I started playing games when I was a kid on a C64/128, never had any of the consoles, moved to DOS gaming and then to Windows 3.1, Windows 95, etc.
One major change in RPGs: The addition of the journal / quest log. In the 80s and early 90s, there was no journal, no indication of what you had learned, or needed to do, or where you needed to go. Since with long games I play for a while, stop for a while, and resume playing later, I lose track of what I was doing if there isn't something to remind me. (I tried taking notes once in Ultima IV or V but couldn't make sense of them later, because they were just names of places and people
).
It seems like games generally tended to get more complex in the 90s and early 00s and then lately some companies have been trying to make them simpler again. I'm wondering if the reason is that games cost so much more and the money is going to things other than coding, or if they think audiences won't want to deal with complexity, or that they can achieve depth with simplicity. I think they did reasonably well with XCOM (I have a few quibbles), but Spaceships sounded overly simple. I avoided Beyond Earth for a different reason - it sounded like they omitted the heart and soul of SMAC when they created Beyond Earth.
Games are bigger, yes, less efficient, yes. Once upon a time they HAD to be small. The Commodore 64 had
no hard drive, 64 KB of RAM, and games usually came on floppy disks (5 1/4"), which held up to
166 kilobytes of data. Some games were small and you could fit several on one disk (if the copy protection was defeated). RPGs generally came on multiple disks and required disk swapping, just like every era's RPGs prior to Steam. In any case, they had to fit the code and data into the available RAM, which generally meant using assembly language, and for big games, reading from disks and/or disk swapping.
DOS only partially alleviated this - it only lifted the conventional memory ceiling up to "640 KB," some of which was used for the system and drivers, so even if you loaded as much high as possible, you were unlikely to get 600 KB. Although games could run in protected mode and access memory beyond the 640 KB ceiling, the code still had to be small enough to load in the first place. That and the small amount of RAM commonly available at the time (16 MB was a lot!) limited any game's size and imposed efficiency restrictions.
It's easier to code games using libraries and engines and high-level languages than by hand-coding assembly (or at least it's easier to learn how to, and easier to port it to additional systems/OSes, and to make it more compatible, etc). The downside is that those all add bloat. Fortunately hard drive space is cheap, and so is RAM (says a person who still only has 4 GB).
I think there were more genre-defying games on the C64, or rather, I think genres didn't exist yet, aside from really broad things like "RPG" and "arcade game." If I name some of my favorite (multiplayer) games for it, I can't figure out what genre any of them belong in except for maybe the last one, which is basically a 4x except there's no exploration: M.U.L.E., Spy vs Spy 1 and 2, Archon 1 and 2, and Lords of Conquest (a strategy game with provinces).
M.U.L.E. has a lot going on - the others are fairly easy to understand. Games have gotten a lot more complicated since then. They also seem more formulaic, at least AAA games, anyways. Indie games are held up as the antidote, but right now the flood of indie games is such that it feels like if I try to look through them to find the diamonds I'll never finish.
Nowadays a lot of games have "quick-time-events" where they show you a prompt for a button and you have to press it within a timer, or at a certain time, or mash it. Something similar existed in the 80s, however.
summer games for example: "The dash is the most straining discipline: the one who shakes fastest right/left is also the first on the finish line..." There were similar movements required in other games, including Spy vs Spy (for combat, if you were in the same room, to hit the other player). There were other events that were far less annoying, of course. You probably don't want to go so far as to consider almost anything a QTE, like "I HAVE TO PRESS SOMETHING TO BLOCK? ARRRGH!"
A lot of the C64 games I tried were ports of arcade games - you had lives, and they weren't designed to be beatable unless you were really really good at them. They never really held much interest for me, but I'm sure they did for other people. I'm not sure if anyone still makes arcade games, but I get the feeling they might do okay on mobile. It's probably not a terribly accurate feeling, since the 'arcade game' is ~30 years out of date with the methods used to entice and entrap players today.
Of the DOS games I've played the most memorable were Master of Magic, Master of Orion II, early FPS Duke Nukem 3D (LAN MP was fun), The Summoning (an SSI isometric RPG set in a dungeon maze), Warlords II Deluxe, SimCity and SimCity 2000, Scorched Earth, Warcraft I and II, and X-COM (although I had not played X-COM until the 2000s).
You can sort most of those into genres fairly easily, unlike the C64 games I named. Some of them were either the basis for their genres, or came out shortly after the genre-establishing game, or were considered the pinnacle of the genre. Those genres have changed significantly since then, of course.
The 4x genre, both fantasy and space, has seen a lot of games. Sid Meier's Alpha Centauri was one of the most compelling not just because of the gameplay but because of the storytelling - the characters, and how they're weaved into the entire game and tie everything together. This lesson doesn't seem to have been grasped, though, or perhaps it just didn't sell well enough for anyone to think it should be a lesson. I'd chalk that up to being a complicated sci-fi 4x on an alien world, though. IIRC fantasy also sells better than sci-fi in general.
For RPGs, most have tended to have little choice which feels meaningful, but The Witcher series especially shines in this regard. Deus Ex and (Obsidian's) Alpha Protocol are others that have the characters or plot react to your actions, and (also Obsidian's) Fallout: New Vegas has multiple factions and lets you work to win a war for one of them or even take control for yourself. Bethesda's quests, by contrast, are almost all on rails (caveat: I haven't played fallout 4) - especially after Morrowind. I'm hopeful we'll see more games following the meaningful-choice model in the future.
FPSes are one genre which I don't play much anymore, except for FPS-RPGs (Tomb Raider, etc). Once upon a time I was pretty good at FPSes. I'm not sure if it's me or FPSes, actually. I tried the new Unreal Tournament a month ago and it seems like people move absurdly fast, I can't see them shooting, if I try to shoot at someone I miss every time, and I can't avoid being shot. OTOH, I tried Red Orchestra II's MP and did perfectly well. Of course, half the players were dumb bots, but the mechanics all made sense (even though bullets were still lethal, there was actually cover and teams and enemies wouldn't spawn behind you, but could sneak behind you, which I did to take out a machine gunner in a bunker at one point). For UT, it may just be the mechanics strongly favoring the best players, and everyone who gets dominated just drops the game and doesn't come back, leaving just the elite players behind to continue dominating everyone else who shows up to try it.
I played SimCity and SimCity 2000 back in the 90s, and when SimCity 4 came out I tried it but found it way too complicated. I've mostly ignored all the citybuilder sims since then. OpenTTD was fun for a while.
Scorched earth is an
artillery game. There are many more, including Worms. Angry birds seems like a single player artillery game with no opponent, just obstacles and targets.
MOBAs didn't exist until fairly recently, and RTSes aren't anywhere near as prevalent as they were in their heyday.
I tend to nope out pretty quick if a game isn't actually fun, especially if it's grindy / has obvious skinner boxes. That tends to include most MMOs nowadays. Sometimes a game isn't grindy until you reach "endgame," and then you find it's incredibly grindy if you want to do any "endgame" content without throwing money at it.
Most free to play games seem to be a waste of time. I'm not going to give them any money. If I was, they'd be a waste of money too. They aren't good games. They're deliberately designed to take forever to accomplish anything. If I wanted that I'd play X3:TC or X3:AP (Hint: I don't).
90s adventure games soured me on the adventure / puzzle genre to the point that I almost always ignore a game in the genre if I realize it is one, without even giving it a chance, and if I do give it a chance, I've noticed that I tend to be far less forgiving than I am with other genres. When I played through Arkham Asylum and Arkham City, I mostly ignored the Riddler challenges. Occasionally I would stop and poke one, but generally I ignored them. That said, I enjoyed Portal and Stealth Inc 2 (but not Portal 2, which just frustrated me).
These days what I want most in a multiplayer game is for it to be turn based and to be pbem or something similar (dropbox would work) to support several players playing in different time zones at different times, to have features to handle players dropping out or substituting in, to have stuff to do each turn that is meaningful, to not take ages to load, to have a lot of depth... Basically, Dominions 4.
I'm happy that Steam has largely replaced disk-based DRM and shit like Starforce. It's a huge improvement, although some games still include SecuROM for some reason.
(Also, a lot of games have tons of speech now. Only a few C64 games had speech, and only a few phrases, and they were synthesized. It was a marvel to accomplish it at all.)