Rock Band 2
It's pretty much the same game as before. There are still four instruments, there is still a world tour, there is still a character customization closet. The instrument hardware is better (which was a long time coming), and certain annoyances with the game, like characters being locked to gamer profiles (which should have just been patched in the first game), have been attended to. There are some new features, but I barely know what they are - an online battle of the bands mode? More competitive play options maybe? Something like that.
What really makes Rock Band 2 worth it is the new song catalog. Fleetwood Mac, AC/DC, Presidents of the United States of America, even Beck and Modest Mouse - this is really cool stuff to play. To be sure, there are some stinkers (who let Linkin Park in here?), but the game's music collection is still a bargain for the price of the disc. If you didn't like Rock Band this probably won't change your mind, but if you did, this one - really more of an expansion pack - is a no-brainer.
Progress: Gave Up