Okay, I've finally been able to affect some mass. My first impressions are rather positive. One thing I thought was especially weak about the first Mass Effect was the beginning, which just dumps you in the boonies of an (admittedly very pretty) alien planet and then leaves you to wander around for a few minutes. Number 2 starts all guns blazing, with your lovely spaceship from the first game getting blown to bits with you inside it, then dropping you in the middle of a battle at the shadowy research project that has resurrected your ruined body.
The process of importing your character from the first game has possibly been a bit oversold - I can certainly see all the hooks where you can fill in the backstory of a brand new character. But it's possible that my previous good deeds with respect to Wrex and Tali may yet have some effect that I couldn't have got otherwise.
Speaking of which, I'm already satisfied when it comes to two of the main reasons I bought the thing: Martin Sheen's 'Illusive Man' casts a sardonically voiced shadow over events, and the very first character you bump into on your first proper mission is none other than Tali (which seems like a bit of a coincidence, but I'm the last person to complain).
I can also see quite a few improvements on the things I didn't like so much about the first game - the dialogue (at least so far) is focused more on the personal than on broad exposition, the cover system has been improved, your comrades seem more dynamic and useful in combat, and I also like the fact that the weapons feel a lot meatier - the guns in Mass Effect 1 just didn't feel like they had any punch to them.
Now, if you'll excuse me, I have renegade robots to shoot.