I feel the need to add to this thread.

I think one of the reasons people don't like Kuwabara a lot is because most people are sheep and since the people in the show often make Kuwabara the butt of their jokes, fans do the same, failing to see the true depth of love and friendship the characters share. They want to be "oh so cool and hip" so they go and bash Kuwabara. Also, if you think about it, bashing Kuwabara, just with his reactions, is probably one of the funner characters to poke fun at. Also, there's the aspects of
A) He's NEVER brought down by it (for long) and
B) He's the only character who won't glare/punch/hurt you in some way when you're nasty to him.

If you pay attention to most YYH fanfictions (which are mostly HieixOC, btw) you notice that many of the OC's are cold and bitter (usually from some dark, dramatic childhood.) It seems natural, then, that they would immediately go to pick on Kuwabara, the big oaf with an even bigger heart of gold. This only nurtures fanfic READERS hatred for the guy and, well, soon we got a YYH fanbase with flames and pitchforks ready to take out the big guy if he even scratches his nose.

Sad, but true. I'm glad there are fans who can see past all the poorly written fanfics and see how wonderful the guy really is. heart