I never noticed the CD number thing until you pointed it out. XP But oh my gosh, that is ugly.
I guess what bothers me most is if you make something tacky or unprofessional looking. And this is almost always done by taking a crappy looking font or overly in your face colours. Prime examples would be things like Utada's This Is The One or Kumi's Gossip Candy.

I mean, in Utada's case, they were probably going for something simple and all, but it just made it look amateaurish. And Kumi's cover was probably to be all fun and hot and summery, but it only turned out cheap...
Other than that, I can deal with just about anything...unless it's obviously just ugly or something, of course, hah (like Ayu's STEP you/is this LOVE? cover

). Even things like bad makeup or over Photoshopping don't have to always ruin a cover, in my opinion. I try to take everything as a whole, so if an artist breaks out a cover in some super-ugly ass outfit, but everything else is okay, I'll probably let it slide.