What I really want is extremely hard to pin down.. I would just put it simply as "atmosphere".
In Baldur's Gate 2 there is such a sense of mystery.. from the very moment you start the game you are plunged into a situation out of your control, you don't have a clue what is happening and have to rely on your party members to get what little information you can. You are then thrust into a massive city which just oozes atmosphere and activities, with people to talk to, things to see, and everyone has their own story. While for the most part you are guided in the story, it still has just the right amount of freedom so as to feel like a huge world open to be explored.
The city is a masterpiece, it feels like home, but there is still danger around every corner.. you think you know it inside out then you find a new area and a new group of people with their own story, voice acting, personalities and quests. There is such a wide range of encounters all squeezed into one area.. taking the graveyard as an example, you go there to confront Bohdi: When you first enter there are typical skeletons to fight outside and in the crypts while you are searching for her lair (even a lich if you are unfortunate to wake it), there are quests to do here aswell, you end up finding her lair, but on the way is a huge spiders nest part way through the level.. there is also even a whole side area with its own quest to be done with Korgan. Once you enter her lair there are vampires to kill, spells to find, parts of the level to interact with.. it's amazing.
So that's what I have missed most.. levels which feel lived in and are packed with interesting stories and characters. I don't want gameplay which is basically "follow this path from A->B to complete the quest", or talk to people you aren't even interested in. I think the atmosphere is much easier to pin down when using an isometric viewpoint though, and I already got a good feeling from watching the latest trailer.