Honestly, I don't get all the hate. Every character in every serious, or semi-serious anime I've seen had a specific purpose. Sometimes it was the over-exuberant friend, or the horribly dick villain (or hero in some cases), sometimes it was the dense (or wanting to seem dense) flaky character... they all were irritating, but their existence was justified by the plot. (Yes, even the "only exists so they can get killed and piss off the hero" ones.) But doesn't their effectiveness at annoying, irritating, and generally pissing you off mean the character was well written and well developed? And doesn't that mean you actually love to hate them, instead of the unimpressive and pathetic limbo implied by being "unlikeable"?
I'd postulate that the popularity of this thread proves that there are no "unlikeable" characters. Only characters we love to hate (mostly because they were designed to be that way).
PS: Can you really fault a character for the voice actor used to play him/her? Yes, it can get irritating hearing those high-pitched squeals from certain characters (typically the "young girl" characters), or if a voice doesn't really feel like it fits (it's surprisingly rare, and sometimes intentional for plot purposes). But... actually, I have no positive spin to put on that, it's just annoying... The point is it's the voice actor, not the character itself.