Rock Band 2 brings tons upon tons of diverse songs to the table--nothing wrong with not changing somthing that works.
It's unwise to go into Rock Band 2 expecting some decisively different game from the original. Even a practiced eye might have trouble distinguishing the two. You're really paying for two things. The first is a huge new selection of songs. Over 80 tracks ship on the disc, another 20 will be available for free in the first weeks, and literally hundreds can be purchased online from a library that has been slowly building since the first game's release. Previously downloaded songs seamlessly integrate into Rock Band 2's primary modes, and you can even import all the original Rock Band on-disc songs into the new game. Examining only the songs on the new disc, Harmonix has embraced the philosophy of all-inclusion. There's something here for everyone, and the music genres represented are all there for a reason – they are almost without exception great song choices.
I can't think of any person to whom I wouldn't recommend Rock Band 2. Playing alone, it offers a suitable challenge for any skill level. Playing with friends and family, it is one of the best social games ever made – a testament to the way games can bring people together to have fun. Sometimes good game design is about dramatic innovation. Sometimes it's about knowing what not to change. Harmonix knows how to do both.