Mass Effect 1 was a good game with a solid concept, but with shaky execution, especially in regard to the gameplay. Mass Effect 2 fixes nearly all of those problems, and takes very few steps back, and those are minor.
The Good:
The gameplay is enormously streamlined. The aiming and shooting feels much more intuitive, and the cover system works much better. You don't have to mess with unusable weapons.
The worldbuilding is as solid as in the first game, and the multiplicity of hub worlds gives an even greater sense of a full universe.
The characters are, as always, fully fleshed out, and it's quite interesting to get to know them.
The sidequests are greatly improved, with unique maps for all of them, as well as additional story material in the recruitment and loyalty missions.
The Not-as-Good:
The characters are great, but there are so many of them (especially with the DLC), and they unfortunately don't get to interact with each other very often.
There's less of a sense of urgency than in the first game; it basically has a few story relevant missions, and the rest of the time you can just go and do whatever. ME 1 did that to a degree, but unlike the geth, who were a universal presence in the first game, the Collectors don't actually show up very often. They're absent from nearly all the side missions and loyalty missions.
Some of the recurring areas feel a bit small. In particular, the Citadel feels much more limited when compared to ME 1. The presidium is limited to one room, and it just doesn't feel like there's enough to do there.
The "heat sink" system is basically an ammo system, which is a step back from the overheating system of the first game. That said, the other improvements to the gameplay more than outweigh the issues.
Basically, most of the problems from the first game have been totally fixed, and the new issues are all pretty minor. All things considered, it's a giant step up from the first game and makes much better use of the potential that the first game had.
Fixes nearly all the problems with the first game.
Mass Effect 1
was a good game with a solid concept, but with shaky execution, especially in regard to the gameplay. Mass Effect 2 fixes nearly all of those problems, and takes very few steps back, and those are minor.
The Good:
The Not-as-Good:
Basically, most of the problems from the first game have been totally fixed, and the new issues are all pretty minor. All things considered, it's a giant step up from the first game and makes much better use of the potential that the first game had.