First off, longtime designer/creator Will Wright will not be working on The Sims 3.  Is this a positive thing? Will the game undergo a good freshening? So far, it certainly looks like it.

The workload would obviously be too much and too stressful, so you still control one household per game. However, your neighbours behave as you do, advancing, gaining relationships, jobs, etc.

The A.I. is reported to be a lot more sophisticated, kind of tying in with the new social venues, such as bars, restaurants, etc. Some of these are little more than facades, however, though if upgraded in the expansion packs, this could cost you some hefty system requirements.

"Moodlet buffs" will take the place of things like the bladder bar, effectively relieving a Sim upon clicking. Hopefully the end result will be a more fun game for everyone this way, but hey, some Sims fan really like the bladder bar and are going to be pissed off about it being gone (pun intended)!

Trait-based character generation: over 80 traits means over 24 million possible combinations for any Sim. Awesome in theory, as for whether or not they pull it off well could be another story.

More item customization is also promised, though won't be as detailed as it could be, which might be a dissapointment to some.

The Wants and Fears system from the last game will be replaced by the Dreams and Promises system. Dreams can be very simple or ambitious things, and they're also optionally fulfilled. If chosen not to be, they turn into promises (like say, to get a promotion), which you're penalized for if ignored.

The game is really being overhauled in a lot of ways, and it seems as though we won't quite know whether these are all for the better or worse until we get our hands on it.  1UP reports the game's release date as being December 15 of next year. Eep.