I hate when people write a fanfiction and change a character too much. I'm alright with AUs and a bit of OOC stuff, but I feel that there can be limits.
For example, I was reading a Ron/Hermione fanfiction the other day, and the author changed Hermione so much it disturbed me. I won't put the link up out of respect for the author, but I will describe what happened. The author made Hermione change her hair and start shopping at Hot Topic and such for a 'new look'. Now, I'm alright with people changing their hair, and I shop at Hot Topic, but how likely would you see Hermione there? Hermione's character was also changed COMPLETELY, to the point where she seems slutty.
Again, I'm fine with a few small changes. But when people change a character's personality so much, they might as well call the character an OC instead of their name from a specific fandom.
This is just my opinion though, and I guess plenty of people can like this. It just bothers me a lot though.