Scale. I don't know if it really counts as a mechanic but I think it's something that games should be designed around more.
Alpha Protocol I think isn't a super great game (as far as gameplay goes) but I think it handles scale in a way that is very good. Barring the intro, you can go about the majority of the game in various combinations of order, and that matters a lot on what you learn as a character, but also how the world responds to you. But the scale of it (Small number of missions and total plot-related characters) means they are able to weave a lot more meaningful differences in your approach.
I love Mass Effect, and it does a really good job overall of making it feel like your choices do have an impact (sometime to an absurd degree, like someone in 3 sending you an email to thank you for something you did in a D-tier sidequest/collectable in 1) but the scale of Mass Effect is still quite literally galactic and the way things can branch and then consolidate can only go far.
Warren Spector used to give GDC talks about his idea of the City Block scale game and it's always fascinated me. I think a lot of games tend to be a thousand miles wide and an inch deep. I'd like to see more go the other way with it (or maybe in somewhere in the middle?)
This is slightly off-topic, but has anyone seen that three hour cut of the original?
Boy, is it not an improvement. Most of it is extended scenes of Joaquin Phoenix trying to bone down with his sister, and them some training montages for him (I guess to justify the ending's absurdity).
@theflamingo352: Imagine a studio set, with it's three walls. The 'fourth wall' is that non-existent fourth wall that WOULD exist, if it weren't a film/television show/etc. It separates the audience from the narrative.
Prey is my favorite Shock-alike to date, frankly. I had a phenomenally good time seeing out the stories of every crew member on the station, and seeing the believable way Talos 1 fits together. If folks like seeing from one level of Dark Souls to another for world immersion, they ought to check out Prey. The inside (and outside) fits together in an interesting and consistent way, with both tons of little detail as well as grand scope. It's fantastic.