Turned into a wall of text, so I'm going to spoiler.
Take your average Bethesda encounter for example.
1. Setting and place: It's going to be either outdoors, in a one-room zone or a large sprawling dungeon.
2. Characters: You're guaranteed to have one of a couple different options: random bandits, random animals or ghouls, random super mutants, random robots or actual NPCs. Bosses for the most part tend to be just like every other enemy, except way stronger and doing way more damage. The only question I generally have to ask myself about a Fallout boss is "Do they have explosives." That's really the only question that matters.
3. NPCs: What do they do? They're either immediately hostile, hostile pending a quest, fighting other baddies or there for you to talk to. They sell and buy, they might give a quest, at some point they might be a load bearing part of a quest. Maybe a few of them will turn into automaton that follow you around and regurgitate the same lines over and over again and carry your stuff.
4. What will you find: Either it's going to be a gun or melee weapon, armor, consumable (food, aid, junk), special collectible (Mags, Bobbleheads, etc...) or quest item. You will have some tepid traps to deal with. Maybe some contaminated water or patch of radiation to move through.
I'm being very reductive of course but those 4 points basically describe anything the game can and will throw at you.
Using Dark Souls as my counter example (which someone will undoubtedly do in a reductive fashion as well):
1. Setting and place: DS doesn't follow the same kind of format so it's hard to compare. But consider dungeons can be large, small or stuck in between several zones and that zones are intimately connected to each other in both form and function. (Think of the Pit of Majula in DS2. It's one zone with a huge pit in the middle and three separate zones each branching off of it.)
2. Characters. Speaking of enemies, their intent and what they do can vary wildly. From slow movers to big huge enemies, paper tigers, fliers, archers, poisoners, exploders, ambushers, cannon-fodder, petrifiers, cursers, bleeders, invisible, dragons, skeletons on a spiked wagon wheel, slugs clinging to the ceiling, huge worms inside walls, ghosts inside walls, stationary ones that emit that emit poison clouds.... Boss encounters are specifically designed to mix it up where they can. Through adds, different attack patterns, different gimmicks, different arenas that can make fights more dangerous.
3. NPCs: While by and large they all just talk to you and sell to you, some trick you. Some need things although to call what they ask for a quest would be too incorrect. The lives of them are important for several parts of the game. Some have no point at all. And they can all die and the game is ok with that.
4. What you will find: of course weapons and armors and consumables. But what do these things do, is the real difference. Some DS consumables don't fit cleanly into buff/heal/cure/thrown weapon/token categories because their possible effects aren't delimited by the other game mechanics the way Bethesda games are. DS traps and puzzles can vary pretty dramatically too, from instant death ones to slow death ones, strategically placed to make them the most painful they can be.
Breaking form is simply this: when a game creates an overarching set of mechanics and rules it can do two things. It can continue to funnel all points of game through those mechanics and rules or it can "break form" and do something that subverts them, contradicts them or creates its own new category of mechanics.
WARNING: BEGIN OLD-TIMEY RANT. Being mid 30s and comparing gaming as I once knew it to now, I feel like a big sea change happened somewhere between Diablo 1 and 2. Games to me went from feeling like every mechanic existed in and of itself, to feeling like it was part of a greater whole. I.e. Diablo 2's loot rarity mechanic. I.e, skill trees. I.e. Quest Giver --> Quest Area --> Quest Reward format that WoW codified. I single out Blizzard so much because they really figured out how to make their entire game part of a systemized set of visible, transparent mechanics. The format is all. Games went from being impenetrable and disjointed like a late 80s party-based D&D RPG to being streamlined and "slick" so just about anyone could pick it up.
And that was pretty rad to experience the change. Games felt like they'd somehow amped up in complexity and scope because suddenly so much content was being created under a system, a rubric, a format, faster than it had been in years past where I think most game play mechanics were each laboriously created without some overarching guidance to really structure design. (i.e, you make a piece of loot. What does it do? Without a system in place to guide design, you create each piece of loot from scratch following just the most basic template. Now switch to the Blizzard format of well structured design. It's got a rarity. It's got base damage. It's got crit chance. It's got attack speed. It's got accuracy. And then it's got a whole host of additional modifiers already mapped out to further add to it.) Games not only felt like they became deeper, they also felt like they became fairer. Game mechanics weren't opaque, nor was the structure. You knew what to expect from it every time, instead of like an older 80s RPG where you'd meet an NPC and be like "Does he want me to feed him, punch him in the face, ask him the right question....? WHY THE FUCK IS THIS GUY HERE?!?!?!")
And that was pretty magical to experience as a gamer.
But as time has gone on I've suddenly been pining for and more attracted to games that ARE opaque, that DONT spell it out, that I DONT know what to expect from when I turn the corner. It feels like many games of this era have internalized this need for an overarching meta framework on which to hang game play on, a big shiny obvious framework the player can see from the outset. Things like.....
-Classic Fighter/Thief/Magic-User archetypes to set up initial character creation. Old games did it too but it was less emphasized as a core part of the game's mechanics.
-A skill tree that breaks up along class lines, and maybe subdivides further into specialization.
-Gameplay that also flows from the three class choices: strength damage and tankiness, "agile damage" and avoidance and spell damage/magic survival.
-Fire/Ice/Lightning as a quick, easy way to start thinking about magic. (Used to be Fire, Wind, Earth and Water but I think people had an increasingly hard time trying to figure out how to make water and earth into attack spells. So they just gave up.)
-A rolodex of quests neatly organized and tons of quests to fill that rolodex. Ostensibly in the name of the world building.
-Loot cleanly divided between common, uncommon, rare, really rare, super-duper rare.
-Randomized loot.
-Specific loot as an explicit exception to randomized loot.
-Teleporting around the game world (even Dark Souls gave in on that front.)
It's why a game like Dark Souls manages to surprise and delight me so often. Maybe I'm just not as familiar and jaded with it as I am with other games, but, I don't see the edges of the box with their games as much as I do Bethesda games or Blizzard games. Things feel unique, distinct and it still feels like pretty much anything is possible. For example...
WoW, Diablo and Fallout would never go from your run of the mill enemy killing zone to a zone of utter pitch blackness and plunging drop offs all around that instantly kill the player. They simply wouldn't, because it breaks form. They believe players don't want that kind of novelty to interrupt their expected experience. They wouldn't go from the norm to, say, an area full of inanimate objects that spit poison at you and give you no experience for killing them, and they are the entire basis of a level. I don't really appreciate the subtle differences and ways the environment contributes to the fighting or the exploring in the Sargus Ironworks in FO4. (Other than the fact everyone has a flame weapon.) Compared to Dark Souls where each area feels like it does its own thing (albeit some of them weakly.) The Black Gulch is a very different experience to the Iron Keep Or Drangleic Castle or Amana Shrine, each challenging for its own reasons, sometimes because of something as trivial as the lighting.
END OLD TIMEY RANT.
I guess what it comes down to for me is the sense of craft in the ideas. And I'll freely admit, I'm playing through Dark Souls 2 right now and some of it definitely feels phoned in. But a lot of it doesn't too. A lot of it shines with actual, unique craft. And that's not to say there's no craft in Fallout et. al. But it's of a different kind. They're crafting things within the well-trodden architecture of the systems they've built up to the point it's a very safe and predictable experience for both them and players. The same way Assassin's Creed has built up a format of things you do in AC games (collect shit, reveal the map by climbing up specific points of the map, looting chests sitting out in the open in broad daylight around the city, tear down wanted posters, etc....) When new games in a series come out, they tend to graft a new module to the pre-existing core. Why remake the wheel, right?
So when a game series (Fallout, Diablo, WoW, even Dark Souls) builds up these systems, at a certain point as a gamer I start needing them to break the form they've carefully been maintaining, so they can genuinely surprise and engage me. It doesn't help that "the form" also entails the vast majority of the game being brain dead easy. That tends to obscure anything novel the games try to do. (Like, if Fallout tried to make a real trap themed level, chances are it would fall flat in vanilla because traps just simply aren't that hard to deal with or survive being hit by. When Dark Souls makes a trap level, it can be pants-shittingly hard for several different reasons.)
Here's another example to cap this long-winded post off:
In Everquest, one of the granddaddy MMOs, there was this zone called The Hole. It hooked up to a, for the time, higher-end dungeon. Just getting to the dungeon through the Hole was an adventure. Long drops off cliff ledges down a series of ledges until you'd reached the bottom of the Hole. Each drop had to be timed right or you'd splat. Combine that with corpse recovery instead of just respawning with all your gear and some durability damage, and the logistics of recovering said corpses became a real part of gameplay. Mobs aggroing throughout just to keep things spicy, too. It was chaotic and brilliant, watching 25 to 40 people try to navigate this zone.
I've barely seen its like since. WoW has some dungeons that definitely have "FUCK YOU" gotchas. But even that is pretty much "herp derp you wiped the raid, restart the encounter and hope you don't hit respawns." WoW would never in this day and age introduce a zone where simply navigating it was a true threat. It'd be too disruptive to the format of kill trash --> fight boss --> get loot.
Or take the concept of "pulling" mobs for example, that started in EQ and was a result of the frankly primitive pathing and AI became a carefully mapped out consideration in WoW and it too joined the "format" of MMO gameplay.
Maybe the thing is this: back in the day game devs did everything from scratch. Games took a while to make but there wasn't hype then as we understand it now, so they could create in relative peace and obscurity. We got a bunch of brilliant, unique gems and, honestly, a lot of trash too. Then games really became part of pop culture and design itself got systemized to facilitate better work flow, faster turn around and bigger scope. We got bigger, shinier games produced faster by bigger teams. But also produced in some sense by committee, where there's fewer original ideas in them because they didn't fit the form or couldn't be rationalized under the budget. And reusability is just a financial reality. If you make one successful game of big scope, you're going to reuse at least 50% of the ideas and assets in the sequel.
TLDR: I feel a game like Dark Souls is closer to older games of the 80s and 90s in terms of how hand-crafted most elements of the game are, and how it's willing to try varied things rather than always sticking to the same format. Versus a game like Fallout which is very much of the modern era and has a form and a format it sticks to that structures most if not all of gameplay. Sorry for all the blah blah but the whole idea of "breaking form" is one that's constantly on my mind as I play games these days. When I get into a fantasy and see Fire Ice and Lightning as my magic choices I just grown inwardly, loudly.