There is a large body of Japanese otaku who actively dislike Hirano Aya. She and Mizuki Nana seem to have the highest negative to positive thread ratio on 2channel.
I usually put it down to two things: first, that otaku have a limited and conventional idea of how seiyuu should sound; and second that they demand that their seiyuus not be "disloyal" and seek popularity from groups other than otaku.
But Watch! Seiyuu-san recently posted an interesting list of reasons from a poster on 2channel for why he hates Aya:
・her face is appearing too often
・no bathing-suit photos
・poor singer, which can be seen in live events when she has trouble breathing
・even though she's a poor singer, she released three singles in three months
・weird song arrangements
・her voice is screechingly high and hurts your ears
・her voice shifts over the episodes when she is playing a single character
・her voice and acting are the same no matter who she plays
・despite her poor acting, she is praised to the skies
・in interviews, she just says what people want to hear (about hair color, about precognition)
・puts on a false voice (then drifts back to a lower one later in the interview)
・hearing her talk about herself is painful
・上から目線で話す（>>6）（「好き」と言われて→「写真撮りますか？」）[I don't understand this line, but I think it begins by saying that she talks as if she's superior to other people]
・fake posts in her blog
・I hate the way she opens her eyes wide for photos in her blog and elsewhere
・her duck-lipped expressions (see photo above)
・her fans are annoying
・I hate KyoAni
・I hate Haruhi and Lucky Star
・The media go on and on about how great she is: appearance, acting, singing, etc.
Anyway, I thought it might be interesting to hear from the "anti-" side. Lots of otaku just can't figure out why she's so popular.
I think there's some truth in the idea that she just isn't as professional a singer or actor as many others. She never did really go to seiyuu school. I think she does better than the standard professionals, but she doesn't have their specific skills. In the interviews I've heard, there is definitely a lot of performance, but I like it when her voice drops and she gets more real. And I really like her blog, which seems no more contrived than anyone else's to me, and does convey something of a real person.
And I do think that, although she feels quite a bit of insecurity, she really is a bit of an ojou-sama, and feels superior to a lot of hardscrabble seiyuu. She has had photos taken in bathing suits, but never particularly revealing ones. And I think she is happy to get jobs with NHK and other high-class organizations.
I was well aware that some photos were not posted in her blog on the days they were shot, even before people started applying Exif Reader and saying that she lied about where she was on Christmas Eve. I don't think she lied. She just didn't say that the meal she was posting about happened a couple of days previously. So what? And duck lips can be cute, lol.
In any case, on his blog, Shiina Takashi, the mangaka of Zettai Karen Children, for which the anime will start in April, says that although he only asked for a "popular young seiyuu," it was really Aya he was thinking of, and whom they ended up getting to play Kaoru. He says that her voice has "an unforgettable enchantment."
He thinks the character has to be played as a tomboy with a feminine voice, a mischievous princess in whom you can hear a future charismatic queen of the demons. And he says the director and the sponsors are pleased, too. Personally, I think this kind of subtle calculation of how a character should feel is right up Aya's alley.