Harmonix clearly expected the Green Day name to make up for lack of content. Still a solid collection of songs.
The game boasts a fair amount of songs. Around 40, most of which are from the more recent years of the band's career. 21st Century and American Idiot are definately the more prominent albums (especially if you've already downloaded some of the band's RB2 DLC). I know a lot of gamers were expecting more of their older songs. Albums like Kerplunk and Insomniac are nowhere to be seen. I personally would've loved to see "Poprocks and Coke", "King for a Day" or even some of their cover songs, like "Twist and Shout" for more variety.
Most of the unlockables throughout the game come in the form of pictures of the band in their early days. You'll earn almost all of them throughout the course of the career mode. There's a few more to be unlocked through the challenges.
What Green Day Rock Band is really lacking is more venues. You play in the same 3-4 locations throughout the entire game. It never really feels like you're playing as the band during their rougher 'on the road' days, and you also never get the feeling of being international superstars. The band's avatars don't even have a nice range of motion. You'll see them come out on stage, stand on their speakers, wave to the crowd, and Tres will occasionally throw a drum stick. That's about it.
Harmonix clearly didn't spend enough time with the band creating a true to life environment for Green Day. They expected the huge fan following to sell copies of the game, and for the most part it worked. Hopefully they show a little more support for the gamers in the form of future DLC. Until then, the $60 price tag is pretty steep. It would've been worse without the ability to export all of the songs onto your Rock Band 2 game.