the biggest selling points for me were: tracklisting, instrument quality, and downloadable content. I would normally just go and buy the new guitar hero game, but how can i do that when rock band 2 has songs like "Aqualung" by jethro tull and "Spirit in the Sky" by norman greenbaum along with hundreds of other downloadable songs. I've already bought the metallica, oasis, and classic rock track packs!
Instrument quality was the second selling point. Although it's cool looking to have the cymbals ala GH:WT drums, you get the more STABLE bass pedal with Rock Band 2 drums because of how it sits on a horizontal bar. The GH:WT pedal has reportedly slid on uncarpeted floors, or flipped sideways. The RB2 drums, are as SILENT as it can get and they are VERY sturdy.
The last deciding factor for me was the downloadable content. By years end, there will be about 500 downloadable songs on the playstation network. If you like Rush, oasis, megadeth, metallica, blondie, lynyrd skynyrd, etc. then Rock Band 2 is for you. Also, the software itself is just PLAIN FUN! to play through. You unlock songs by earning cash, fans, and hiring staff. The customizability of the characters is also tremendous, to the point where you can change eye colors! I hope this review helps, and don't pass up a fantastic game!