There is something to be said for retaining some of the more complex mechanics. It can give the player more options and choices.
Take Daggerfall; you can give your character advantages and disadvantages like those present in Cataclysm. Things like "Takes damage in holy places" or "Forbidden Item Type: Axes" or "Regenerates health in Darkness". While these have a minor effect on gameplay they can help improve the narrative the player is developing for their character.
I had one character in Daggerfall with a custom class "Black Guard". They couldn't wear leather or chain armour, mana drained when outside during the day, health regeneration in the dark and was restricted from using iron and silver types. Advantages were higher critical strike, immune to disease, possibly absorb magicka.
The enchanting and alchemy systems in Morrowind are easily broken if you know how to but I do like having a system that you can exploit as it can be a good reward for reaching the top level. I have a mod for Skyrim that removes the item specific requirements for enchantments and I can make a suit of clothes that removes all the mana cost for conjuration. Battles involve me summoning endless waves of skeletons to wear any opponent down. That's pretty darn fun, it'll get boring eventually sure but then I can move on to something like endless waves of fireballs or another game until I feel like playing Skyrim again.
Well, then there is the neutral group that likes 2 or all 3 of the games equally. I think all the games in ES series are fun (at least starting with Morrowind, there is of course older ones)
Morrowind I love just because straight out its a good game. No need to mod it, and if you do...you don't really need an overloaded mod list. The alien (but small) land. The main story is epic too from the bit I got into.
But what I like most about Morrowind is when you talk to NPCs, they actually have information for you (to read). And I don't need to mod out fast travel like I do with oblivion/skyrim because there is none (except striders).
Oblivion I like just because its straight up fun. I love the intro quest through the sewers (vastly better than skyrim), but most hated that *shrug*. The guild quests are epic, and outside of adding new lands and some quests and maybe a gameplay mod or two...don't need any mods at all (not for me anyway). When I play oblivion, its with 20, maybe 25 or 30 mods at most...and I just have a ton of fun. World is kinda cliche, but I still love it. It really feels like a friendly land, but filled with danger. Plus the music is epic. Huge negative is the blob characters. Not a big deal to me, I think its hilarious, especially if you like and find b-rate horror movies hilarious.
Skyrim, even though I like it as much as Morrowind/oblivion...I have to mod the hell out of it to be any fun. I can't play vanilla at all, its so lacking. Plus I have to get mods that actually give description of where quests are, cause I always turn off the lame quest markers (Skyrim nerfed quests SO much, huge fail here) and obviously turn off that fast travel. For skyrim to be any fun for me, I have to have 200-250 mods...but once that is done and working 100%...the game is a blast, and as much fun (if not more) than oblivion. I feel vanilla skyrim lacks a lot of what I liked about oblivion, but modded it becomes A LOT better. I like being able to join thalmor (with mods), kill douche ulfric and corrupt imperials and become high king.
A lot of the Skyrim mods (even though newer) are vastly more advanced than they are compared to any oblivion mod today. Some Skyrim mods are amazing. SkyAI, SkyTWEAKS, playable dragon, driveable ships, build own town, crap a ton of stuff (though vanilla does that anyway, but mods makes crafting even more realistic)...so many mods that are just so advanced compared to anything oblivion came out with.
Honestly, if I was forced to choose...and this is my personal experience.
Oblivion/Morrowind tied for a vanilla (or very close to it) experience. Both are very different games, and good for different reasons. Just because both are "Elder Scrolls", they are different and can be enjoyed for different reasons.
Skyrim goes dead last for vanilla (including close to it), but goes straight up to #1 if heavily modded
I know this is a Morrowind topic. But a lot of people always want opinions on the 3 games.
As for people curious, I play Skyrim most. But I enjoy oblivion/morrowind a lot.