...I'm a bit dissapointed in the interface. Specificly with regards to how it shows completions.
Right now, if I finish a song on the drums on Expert with five stars (let's just say), that doesn't really mean I've mastered the song. I may have only completed it on beginner with the guitar or not at all on the bass, and so on. Unfortunately, there is no way to know that from the interface given. All you know is that you've five starrred a song on some part of it.
Which is too bad. If I had a chart with all the instruments across the top (guitar, bass, drums, vocals 1, 2, and 3) and a difficulty rating/star rating listed for each one of these on a per song basis, this game would have had me playing for months on end to master each individual part.
I know this is a party game, but with this simple change you could also have had the solo part of the game be rich and rewarding. And I suspect that this game is played solo most of the time (becasue really how often do people have "Beatles Rock Band" parties? As it stands, once you finish the story mode one time through, it's not very interesting after that.
Anyway, I'm going to make such a chart myself with Excel and keep it on my desk. But it really ought to have been a native part of the game. Just a suggestion for the next Rock Band game (perhaps Green Day?).