They fall in love way too quickly and it seems fake because no one falls in love instantly, especially teenagers.

I actually agree with this one. I've never believed in the whole love at first sight thing. It just seems rather impossible to me, but maybe that's because I'm more attracted to a persons personality and you cannot possible find out everything about them instantly. They do fall in love way too quickly, but that doesn't mean that has any effect on how they love each other throughout the rest of the books.

Especially Ginny and Harry.

I feel like both were a bit rushed, but mostly in the movies. Which is more understandable as they have a smaller time frame to work with, where as an author is free to write as many pages he or she so wishes.