There are three major things about Rock Band 2 that peel my potatoes with paring knives of ANGER:
(1) The UI design. How does something that horrible make it out of QA? I don't know what's worse, the ninety minute, Trance Machina-esque, "we're going to lock up your controls until you see the WHOLE ANIMATION" tweens, the stupidly long loading times to get back to the, uh, MAIN MENU, or the fact that you have to hit the red button about fifty gazillion times to go back to the main menu from every screen.
(2) Harmonix's concept of individual user preferences (or lack thereof). Why should I have to pick "EXPERT" for the instruments every single goddamn time I start playing? Is it really that hard to say "Oh, Spectere84 last played expert drums and expert guitar/bass...maybe we should save this so that he doesn't accidentally select medium guitar and be bored out of his mind for four minutes." Also, why should I have to go through the menu every single time and select "breakneck speed?" Furthermore, why can't I pick breakneck speed while my friends, y'know, play at normal speed? And finally, who's bright idea was it to set the note scroll speed to that of the player with the lowest difficulty? When my friend is playing easy drums and I'm playing expert guitar, it's almost impossible to read any song rated 5 and up. Is it that big of a deal if my notes scroll faster than his? Seriously, that's complete bullshit and horrible game design. It either winds up alienating the good players (i.e. I can't play on Expert because of an unnecessary game limitation) or the newbies (my friend either plays an instrument that he's good at or is forced to use breakneck speed to make my chart readable).
What pisses me off the most is that this kind of stuff is nothing new. How long as DDR allowed individual players to select different difficulty levels and speed mods again? Since 2001? There's absolutely no excuse.
(3) The shitty instruments. Oh. God. Seriously, who designed that shit? Between my friend and I we've had to return several instruments, some that had issues right out of the box. I bought a wireless guitar controller for the original Rock Band, went through three of them, then finally gave up on it and got as much money back on it as I could. I've had issues with both of my drum kits dropping hits, I've had the bass pedal on my RB1 set snap. My RB2 guitar has very shaky solo buttons on it; sometimes they just stop responding altogether. About the only good thing is the mic. Let's not forget the guitar's nasty tendency of either not activating overdrive or activating it at inopportune times.
I know it's not impossible. I own a second-hand Xplorer GH2 guitar that works beautifully. The buttons are very responsive and the strum bar is fantastic. There is absolutely no reason for the instruments to be so unbelievably awful. What did I pay almost $200 for again?
