Why fix something that isn't broke?
I just don't understand why. There wasn't much wrong with the first game. Sure I felt the sequences driving the Mako got a little old before the end, but getting the sense of exploring a moon and seeing the huge parent planet in the sky was darn sweet. Obtaining Stiletto pistols I through 8 wasn't particularly inventive, but it still got my scavenger instincts going. All in all it worked, and they all just ditched it!
It's dumbing down plain and simple. I HATE cover systems, and Mass Effect 2 shamelessly copies Gears of War's, warts and all. This means obvious kill zones littered with concrete barriers and bullet proof boxes and all manner of other crap to put your back to. Health and shields regenerate after a few moments of not getting hit. Just like all the other shooters.
On to the story. It seems the writers that worked on ME1 went on to write Dragon Age: Origins and someone else wrote this. The main story is just minimal. ME1 even though it was somewhat short, still had a bigger than life story. ME2 just doesn't really try at it. The bulk of the game is spent assembling your 10 person team. Now these characters are great. Fine, but if the story isn't stellar, the gameplay better make up for it. And it doesn't really.
I'm being harsh here; the fully emoted and voiced over conversations in ME2 are probably even better than in ME1. Fantastic. But that's flash, not substance. Don't expect an RPG, don't have an exceptional game, and you'll probably have fun.