It improved upon nearly everything that was in the first game, and in many ways, it is still better than the 3rd and 4th games in the series.
To those who say ME2, gameplay-wise, I'll give it to you, but story-wise, not so much...
Personally, I thought pretty much every squadmate got plenty of character development over the course of the game. My only problem plot wise with ME2 is that ultimately, it contributes like 5% to the overall trilogy, though this is something I fault the writers with. You could honestly put Arrival right after ME1 and not too much would change besides Cerberus' involvement in ME3's plot.
Playing: Half Life 2, Dark Souls 2, Vampire The Masquerade, System Shock 2, Borderlands 2, Mass Effect 2 PSN: T-Snake94, XBL: T-Snake 94, Steam ID: T_Snake94