OK, so I got a laundry list of stuff. I play on 360 but I'd imagine this affects all versions since they aren't really bugs. The first thing I started a thread about so I'll copy/paste most of it.
I'm having real issues with the new fretboard in RB3. Despite them adding a solid texture to it, the stage antics show through WAY more than they did in RB2, especially at the top. I also think the notes are a bit darker, and hopo notes are smaller. If they aren't smaller then the darker color makes them look smaller. And because they're darker, all the flashing that's going on makes them disappear to my eyes, especially green and orange. Also, it seems like breakneck speed doesn't space out the notes as much as it did in RB2.
It all boils down to that it's much harder for me to pick up the notes than it was in RB2, especially in fast hopo runs. I think if the top of the fretboard (which is where I fix my eyes) was made less transparent so that it actually blocked out the background it'd help immensely. As it is my eyes can't pick up the notes when they first appear at the top most of the time, so once they get down to a point where they register to me, my timing is now off because I'm seeing them later than normal.
So basically: Make the fretboard less transparent
Also, see the guy above me's post.
The other big one for me is breakneck speed not working in practice. It's kind of hard to practice something when it's not going the same speed you play at.
The last stuff is minor knit-picking stuff but I'll post them anyway.
This might actually be a bug, but the Jackson Custom Kelly with reverse neck guitar does not seem to let you add art to it properly. When adding, the art appears on the 2 small pickups under the strings. Every other guitar I tried has the art appear on the body (and art appeared on the body of this Jackson in RB2). Hopefully this can be fixed.
The Good Times wrist item does not appear on some clothing you'd expect it to. I'm pretty sure in RB2 it appeared over top whatever torso item you were wearing, regardless of sleeve length. In RB3, it only displays for certain short sleeve items, which would be fine, but even ones you'd expect it to it does not. For instance, the Alabama-Slamma shirt has it's sleeve's rolled up, but the Good Times wrist item still does not appear.