Rock Band 2 brings the Wii what the first game was lacking: online gameplay. Rock lovers, this is your game!!!
Rock Band 2's tracklist probably isn't quite as exceptional as that of the first, but there are still twenty more songs on the disc and since there is online play now, you can buy as many songs as you want off the Rock Band music store so the quality of the on-disc soundtrack is kind of a moot point. (Oh! and Wii users: make sure to use an SD card to store all of your DLC so you don't take up all of your Wii's storage space.)
For those who don't have online access during gaming don't worry, Rock Band 2's online gameplay is not its only highlight. Although the soundtrack might not be quite up to the level of that of the earlier game, Rock Band 2 for the Wii has caught up with the same game for the XBOX and PlayStation. It has world tour, characters can be created, and there are additional extras like no fail mode (world tour is the most significant feature). It feels like a much more complete game than the first Rock Band.
The biggest complaint I have of this game is its inability to transfer songs from the first game to its own soundtrack - largely due to the first's lack of online play. Although this can be irritating when you are unable to play in battle of the band tournaments that contain Rock Band 1 songs or when you have to switch discs when you want to play the first game, it is only a minor drawback. Also, I'm still somewhat averse to the drum design. They are very clunky and noisy, and therefore not as much fun to play as guitar (maybe that's just my personal preference as an actual guitarist, though). But, I must note that the drums - especially the bass pedal - have been drastically improved since the first game. Lastly, I hate how the scroll speed slows down to the speed of the player on the lowest difficulty level playing, which really messes up your timing if your playing on expert or even hard. This makes playing with friends (if your on different difficulty levels) frustrating... but this problem existed in the first game as well.
My other complaints are pretty trivial, regarding minor subtleties of the game: bass scores aren't displayed in quickplay, online leaderboards sometimes are slow to update, characters can't play two instruments at the same time (guitar and vocals, for example), etc.
In conclusion Rock Band 2 for the Wii is a superb game. I've played it for hours on end and it NEVER gets old. The addition of online gameplay makes this game much more addictive than the first. When you get bored of the old songs, you simply download some new ones. And if you don't have any friends over to play with, you can just play with them online. I don't think you'll ever need to get another Rock Band because this one encompasses so many features... Plus, Wii recently caught up with XBOX an PS3 in DLC so now you have even more music to keep you occupied.