First of all, I am in love with Rock Band Blitz, it's been a blast so far and I will probably keep playing for a long time.
One thing that bugs me, though, is the way the songs are ranked by difficulty. I'm not even starting with how inconsistent these are on traditional Rock Band, but it seems that to use the same method of difficulty rating for Blitz isn't helping anything.
I'm sure "Moves Like Jagger" is pretty hard to sing (haven't tried yet) but to say the vocal track is a tier-7 difficulty on Blitz doesn't make sense.
Difficulty on Blitz should be ranked in other terms, like density of notes or if the song has four or five instruments.
That's what I think, at least.