There are a lot of great characters in Harry Potter. I love a good portion of the Harry Potter characters. However, J.K. Rowling writes very realistic characters, which means no one's going to like every single one. It's like in real life. Sometimes you just don't like certain people. There may be nothing wrong with them, but for some reason or other, your personalities just don't fit together.
One of the characters that I feel this way about is Cho Chang. When I was younger, I didn't have a problem with her, but as I got older and re-read the books, she got more and more annoying to me. The way she acts around Harry in Order of the Phoenix drives me up a wall. Who goes up to a boy the like and starts talking about (and crying over) their ex-boyfriend? I understand she'd want to talk about it and thought Harry would understand, but it's not really something you just spring on a person. I also think she completely overreacts on the date. Yes, Harry is insensitive and should have worded things differently, but she doesn't let him give an explanation before storming out the door. It kind of annoys me that she just comes up to Harry and expects him to forgive her (which he does) later.
As you can see I don't have a very long list of complaints about Cho. We're honestly not given that much information about her. Maybe if we got to know her better, I'd actually start to like her. We'll probably never learn more about her (unless there's something on Pottermore) though, so I guess I'll never know. From what we do learn in the books, she seems very annoying to me. I don't think she'd be someone that I'd get along with very well.