Nickelback, 30 Seconds To Mars, 50 Cent, Just about any emo/screamo/metalcore band, any 'pop metal' (Disturbed...), Linkin Park, Good Charlotte, Jay Sean... To me, these artists cause near-physical pain with their terrible music, public personas, and/or image.
I'm a firm believer that you can judge an artist based on their public personas. 50 Cent, for example, tweeted something along the lines of 'if you're a man and don't like vagina you should just kill yourself'. Sure, Kanye has done similar stuff, but Kanye has a lot more talent than ol' Fitty C to back it up.
I also really agree with the ideas that mainstream pop music is a deceptively diverse landscape, and you'd be foolish to lump all pop artist together as 'talentless'. Artists such as Lady Gaga, Beyonce, Kelis, Robyn and La Roux are clearly in a different league than the likes of Ke$ha and Katy Perry, despite similarities in genre and popularity. Pop stars like Rihanna, Justin Bieber and Gwen Stefani, though not exactly stellar, still have distinct advantages over the true scum that is Ke$ha (particularly Gwen, thanks to No Doubt). Even if pop is not your cuppa tea, and even if you think Lady Gaga is a hideous meat wearing transvestite, you would be ignorant to call her 'untalented', because she has a truckload of talent.
I think it's just a case of 'the more 12 year old girls like it, the more you want to hate it'. Justin Bieber, like Twilight, though neither exceptionally good nor bad (both are exceedingly average in their respective fields of media) are given a lot of hatred almost simply to balance out the exaggerated fandom that both receive. Both are average, and honestly the majority of the time I'm ever hearing about Bieber or Bella is when some guy is having a cry about how terrible they are. It's average stuff, and it would be better ignored than perpetuated through criticism? The same applies to a lesser extent to most pop music in general. Just my thoughts on it.