This will be a rather short review for me since with any rhythm game, there are only a few aspects that need to be reviewed.
The set list in Rock Band 2 is a great improvement over the original. I never really cared for the original set list in the first Rock Band. There just didn't seem to be enough variety. This is definitely not a problem in Rock Band 2. There are 84 tracks total ranging through every decade of rock's life span. With such a high level of variety, you should not have a problem finding at least a few songs you will recognize and enjoy.
If you have played Rock Band, then you will know what to expect with Rock Band 2's gameplay. For the most part its identical. But Harmonix has made a few much needed improvements like online world tour mode and battle of the bands. The only real disappointment I really had with the changes they made is the exclusion of the standard single player mode. I know they are trying to force everyone to play world tour mode (which is a very fun game mode), but I still miss getting to play it the classic way. Plus, it can be a very boring and irritating process to find and unlock every song in the game.
Just like in the first Rock Band, the graphics in Rock Band 2 are beautiful. Of all the rhythm games on the market, Rock Band 2 leads the way in graphics. The venues look incredible and the characters and instruments are mind blowing. The gems on the fret board still look the same, but who really cares how they look anyway?
Overall, Rock Band 2 is a great improvement over the original. The songs list is very well done this time. I wanted to give this game five stars but there are just not enough improvements to the game to make it feel like a full-blown sequel.