I have now rated most of the songs here (well, those I can remember the charts of) and I have seen a strange patern in the game when it comes to guitar and vocal songs:
Guitar: Most of the songs is pretty mediocre for the guitar, with some truly terrible and truly epic songs here and there. They should have picked more songs with guitar solos, aswell as more challenging ones probably.
Vocals: There is ALOT of great vocal songs here, and I found myself rating way to many with 5 stars. Truly a great game for vocalists atleast.
That is what happens when you have to much pop and to little rock possibly: It stays great for the vocalist but not so great for the guitarist. More variation next time please, Harmonix. Still, the game is great on drums, bass and keyboard for the 50 songs that have a proper track for that (I don't count songs like Before I Forget as having a proper key track).