So, I hate Ginny. Absolutely despise her character and everything she represents, really. Basically, everything is her fault.
Alright, I admit that was an exaggeration. But, seriously, I blame her for a lot of things. Here's my problem: she is the motivation Harry needs to give up being a hero, and I never wanted him to stop being a hero.
Harry Potter is our hero and we love him for it. He is single-mindedly determined to stop Voldemort and avenge his parents. He is surprisingly empathic (empathetic? I'll never be able to decide that one) and loving and good. Dumbledore is blown away by how awesome Harry is and Voldemort can't stand against such goodness. I mean, seriously people, when confronted with the fact that the only way Harry can possible enable the defeat of Voldemort is by sacrificing himself to Voldemort he goes ahead and does it. Alone. He is famous and noticed by everyone and inspires heroics in those around him.
So, in this context of Harry as a hero, what role can Ginny possibly have? She's the cute girl who thinks he's amazing, not because he's Harry Potter the hero, but simply because he's Harry. She doesn't need him to be defeating Voldemort to earn her affection. She doesn't value him as a hero, she values him as a person, while us readers want him to be a hero.
Compare her to Cho (who is hardly perfect either, of course). From the earliest scenes when Harry has noticed Cho, she is inspiring him to go out and be a hero. To him, she is something unattainable, and if he could just do something amazing enough, he would earn her attention. Cho could be Harry's Madonna, his Dearly Beloved, his Beatrice, or any of the countless other muses that inspired greatness in the men who worshipped them but couldn't actually be with them. As a girl in Harry's life, she can help him to bring out his heroics.
Harry needs to have both of their influences in his life. Ginny is needed to remind Harry that he is a good and like-able person, which is valuable. While, Cho is needed to remind Harry that he is a hero and that is valuable as well.
In book five, Harry stops being a good and like-able person for awhile. He allows the pressures of being a hero consume him and bad things happens because of it. Notably, Cho gets much more of the focus in that book (although Ginny certainly gets attention).
In the end, though, Harry chooses to give up his life as a hero. He settles down with Ginny. He does this knowing that he is giving up his life as a hero. He even admits it. In both books six and seven, he describes being with Ginny as living another person's life. At the end of book seven, with Voldemort gone (oops, did I just spoil something...probably not) he is finally given the opportunity to permanently live that other person's life, and he takes it.
And it's all Ginny's fault. And I hate her for it.
That is all.
PS - Yes. I'm an asshole to a fictional character. I feel terrible, can't you tell?