Well, I don't like...
-too stereotypical personalities.
-personalities that are described with just one word
-absolutely "perfect" like a superman, no flaws, no weaknesses etc. None is perfect IRL (likeable characters has usually one or two flaws/weaknesses)
-I'm OK with "dere" types as long as their personality isn't just about being a ___dere (the only dere types that creeps me out are yangire and yandere, others are more OK IMO)
-In UTAU case, I don't like if their personality info is as long as a novel (UTAU is a singing synthesizer at the end and not a comic/manga character, so why it should have a super long bio at the first place???)
-disliking/loving a other Vocaloid or UTAU* character that isn't your own
*it's fine if the other UTAU is your friend's and they agree with you with that setting (being rivals, lovers, etc.)