Buffy didn't like Spike because he was a vampire, plain and simple. That's the short answer. It explains why she didn't like him, however in order to *know* (or at least, gain an understanding of) why she treated him like dirt, one must look beneath the surface.
Off course, from what I've gathered from this thread (and I haven't read all of it yet), it's pretty obvious for the most part. Mostly for the reasons that Shadow Weaver listed. I like what he said, he articulates so nicely.
I think the number one reason would have to be (at least, in the sixth season) simply because she is pissed off because she was brought back from heaven of all places. I think that anybody in her position would be understandably miffed, I mean who can honestly say that being torn out of a state of perpetual bliss wouldn't incite ire in them? So she is justifiably pissed, and simply lacks a healthy outlet for these intense feelings. A lesser human being would probably have considered suicide as a means to get back to such a peaceful place.
But it may even be that her attraction to him is strictly a physical one, and this probably disgusts her. So she acts out in anger. Maybe that's all there was to it. She likes sleeping with him but dislikes him as a person (err, vamp) and so she uses him, and this causes her grief, even if he is a vampire. And because he is one (and she allows herself to even go there with something she considers evil).
As far as the rape thing goes, I don't know that I honestly buy it. That Spike genuinely wanted to violate her sexually and thoroughly enjoyed it. Obviously he did it, and yes the act itself is reprehensible, no doubt about that. But what I mean is (and keep in mind I actually have not watched this particular episode in over a year), he didn't seem to be into it. He really appeared to be very confused as he was doing it. This could be in part due to the fact that Marsters wasn't feeling this scene, or perhaps because Spike knew what he was doing was wrong but he couldn't stop himself (he's a vampire). Its not as if she struck his sexual fancy and he decided to rape Buffy Summers. This didn't seem to me to be (and off course all I'm basing this off of is movies with rape scenes) one of those "omg I'm a sociopath this broad is turning me on I'm going to rape her lolz" kinda things. I really do believe that this was honestly not his intention.
That whole concept of forcing her to love him this way was simply misguided and twisted, and he knew better.
He had reached a burning point. Nothing justifies it (obviously), it simply explains the situation. It's not as if this had been his plan all along. Hell, she'd already slept with him.
Just think about all the times she treated him badly. Also, bear in mind all the brownie points he'd earned with her, intentional or otherwise.
Their little semi-truce in S-2, the fact that he liked her, believe it or not, how he always looked after Dawn, that time when he refused to tell Glory what she wanted to know about the key, etc. Not that any of this can cancel out what he did; simply that she took advantage of him repeatedly. She forgave him when he had a soul, like someone else mentioned. As soon as she saw that she was able to recognize all the good in him, she viewed him under a totally different light and was willing to look past all the shortcomings their sick little relationship had had.