1) His best hits songs seem to be done/by with already-established artists. His solo talent does not impress me at all; in fact, it annoys me. I hate annoying music.
2) No diversity; limited range. Worst of all, he's doing R&B/Rap+Pop songs.
I'm going to be blunt (at my own peril): Pop music is sucks ass right now (I have my reasons, but they are not relevant here). 8 years ago, there were at least a couple of songs I could get behind; none of that exists today, and Bieber isn't helping anyway. Given the nature of the problem, unless he turns into Josh Groban, I don't think he could help if he wanted to anyway. Besides, he's making a fortune right now; why change anything?
Bieber was fighting uphill to win me over before he even started.
3) He's a living fad; a marketing star.
I have absolutely no reason to care about him, yet his popularity and marketing presence is so immense that it annoys me greatly just due to the regularity he comes up. He isn't Michael Jackson reborn, yet the amount of attention he's receiving now would have you believe otherwise.
Do not be fooled; his talent does not even come close to the hype.
And because the internet will murder me for daring to say this without this obligatory statement; this is primarily subjective. So don't take it personal.