Note that I wasn't talking about fiction, MyD, but mechanical depth. The Infinity Engine games had very little customization and very little interesting character growth on a mechanical level.
Ahhhh, you mean on the character-building (leveling-up) aspect...
It made having a party bearable since not much could be "done" with them.
The 2nd Edition D&D Rules are definitely not as flexible as the 3rd Edition -- you really could only do so much w/ those 2nd Edition rules, w/ your character. Lots of restrictions and whatnot.
I prefer the 3rd Edition rules, myself -- way more open-ended, for building up your character's skills, abilities, and whatnot.
Mostly that's a bad thing for games, but it worked with those.
I think that's b/c you can have a huge party, in which the NPC's are basically most of the important classes in the book. So, you'd have a party with a huge balance, by the time you got SIX in your party -- say a Paladin, Fighter, Mage, Thief, Cleric, Ranger, etc etc. And by the time you reached end-game and didn't do a hell of a lot of side quests, still all your characters will be very powerful and specialized. Everything base is covered.
Normally in RPG's, with only one character, if you have an Open-Class System (unlike D&D), it'd really take forever to become a very Powerful Jack-Of-All-Trades. That means normally doing WAY more than just the main quest. Morrowind and Oblivion's a perfect example of this.
It didn't work at all with Dungeon Siege because the gameworld was generic as fuck and everything felt meaningless.
DS1 and DS2, the world felt like your usual fantasy settings. It was just there, basically -- and unfortunately.
DS2: Broken World was much better, in creating an interesting world. With most of the world destroyed (and you, the Hero from DS2: Original being blamed for it), the way your party treats you (some like you, some hate you), and how other NPC's treat you (mostly like you're a nothing), it's much more interesting in the DS World than every before. It still is an action-RPG w/ the heavy action focus, but it finally had an interesting world and atmosphere (compared to what it was before). It was much better than feeling like these NPC's were just giving you quests and nothing more.