Story wise ME2 is a nice step up from ME1. It takes what ME1 gave us and really developes it into a full universe.
Combat wise it is a huge improvement over the first, and not just from a technical point. The classes are far more balanced and the faster recharge (but one power only) is a big improvement over ME1.
ME2 improves on ME1 in everyway excpet the skill point system, which I really missed at the beginning, but now that I've built my character up it basically has the same number of skills as my old ME1 character it just takes you time to get to everything.