The best relationships (imo) are the ones where you are best friends and really love each other .. the two things compliment each other nicely!
being friends first can pose problems, but simply because sex does change things! not necessarly in a bad way, but once you've entered into that arena you can't go back .. and trying to do that will really mess things up! if you are able to stay cool about it, then it can be a plus to the friendship and you can be a little more intimate without it being wierd (simply because you've been closer than that before) .. and if you try to ignore it, intimacy, which is a part of friendships between boys and girls, will suddenly become a negative thing and then you distance yourself to avoid it, thus ruining what you had..
it's also important to realise that love can change character over time .. love can turn into a close friendship, or the other way around .. but since feelings are involved people are more vulnerable! and more vulnerable means more hurt if things go bad! the reason you can hate an ex (I think) is becaues you loved him/her and s/he hurt you (be it intentionally or not), which is infinitly more painful than if love wasn't mixed into the equation.
For me one of the most important things in this regard - is respect .. if you love someone and her feelings change, it's gonna hurt, but you have to respect the fact that it's not the same for her anymore.. talk it over and do what's necessary, but don't get mad or angry.. because you'll lose alot more than a friendship that way!
Hope that was useful (even if most of it is simple common sense apparantly I like stating the obvious)