Another thing to keep in mind is that for some reason most people on the internet view ANY debate more like a spit in the face argument than a structured debate, no matter how nicely or concisely you put it. Generally, if you join into a debate that is longer than a few posts, you'll take negative rep.
Also, people will hit your rep if they think you've been hitting theirs. Most of my negative rep came out during three threads where I was arguing a point. Most of it came out after other posters made slighted comments about me hitting their rep, which I hadn't done (at that point at least). The only time I hand out a negative is if someone is in a debate with me and purposely overlooks a point I've made or just gets flat out hostile. I'm much freer with the positive, but I think I'm just old fashioned.