Beat Mass Effect 2. Very, very, very fucking good. If Mass Effect 3 improves over ME2 as much as ME2 improved over ME1, it'll definitely edge out Diablo II for bestest game ever.
The rest of this post will be a mix of gushing and minor critiques relevant only to people who have played the game and have too much free time.
The combat overhaul was fantastic. Great shooting. I would have liked a slightly greater emphasis on squad tactics, since all the mechanics are there for flanking maneuvers and stuff, but other than that, everything's great. I'm not so enthusiastic about the changes to the classes and powers. In ME1 biotic powers were like "you get one shot and then it probably won't recharge for the rest of the fight but that power is really something;" in ME2 the powers recharge crazy fast and don't do shit. Combined with the inability of ME2 characters to use all types of weapons, I initially chose to reincarnate my Shepard as a biotic user because I spent most of ME1 wishing I had more biotics at my disposal, but much to my dismay I discovered I couldn't use assault rifles and the biotics were crap. I'm okay with the new set-up, it's just annoying that they made a major change and then expect you to make a decision at the very beginning with no warning of the change and the decision is irreversible.
The graphics are way sexier. Shit looks good. One problem, though; the pre-rendered cutscenes look like ass because they're at a lower resolution. Thanks, guys.
The whole thing is well-acted and well-written. All the characters are at least moderately interesting, although some of them are not well-developed at all (Thane, Samara). I felt the story in both games was a bit of a weakness, in that it was cool and well-written but not at all suited to the free-roaming structure of the game. Hopefully in ME3 this will be addressed.
Another big improvement was more populated areas. Specifically, 3 medium sized ones as opposed to one, but it makes a big difference. Hopefully ME3 will expand on this. Having every mission have it's own area with a unique layout as opposed to the identical bases in ME1 also made the whole world feel much bigger.
The vehicle sections were terrible in ME1; atrocious physics, terrible graphics, absurd landscapes, and completely unfun. Eliminating them was a very good choice.
Okay, the coolest thing about the game. I played with my character from ME1. I played a female character (because if I'm gonna be looking at an ass in tight spandex for 20 hours, it should at least be a female ass). I'm not a hardcore roleplayer, but if the writing's good I can get into it. In the first game I tended towards the neutral option but in the second I felt it was fitting that Shepard be more forceful, in either direction. I felt like I had a lot of control over the path my character took, and yet I also felt like my character had a distinct personality and all the options that I took or could take were consistent with that personality. I think Bioware did an amazing job in that respect, and even if a lot of people don't realize this they will find themselves getting attached to their characters.
I can't fucking wait for ME3.