For me, IRL I like to argue, but here I never (well, rarely) go into GD, simply because it’s easier to argue/debate IRL. Why? When people debate here, it’s so much easier to be proven wrong and/or look like a jackass. And when you look like a jackass here, there’s a permanent record of it. It’s much easier to shoot first and ask questions later in a RL debate/argument. Here if you’re wrong, you’ve got 15 posters posting 8 links each telling you why you’re a complete moron.
Plus, I’m a journalist. I spend all day researching, writing, interviewing, editing, etc. When I come here, I really don’t feel like doing too much researching, writing, etc. I just like to be entertained and learn a thing or two.
And like ultrafilter said, I learned to write formally, and often times I pore over my posts, trying to make sure they’re “publication-worthy.” Granted I do fire off posts without much thought, but the longer ones I often overscrutinize what I’m writing-- which often doesn’t make for very conversation-style posts.
Plus, here I’ll often write a response to something, then realize I don’t really want to say that, whereas in a RL conversation I might just blurt something out without much thought. Again, it’s because of that whole permanence thing.
So why do I care so much how I come off here? It’s not because anyone here knows me IRL-- hell, I may go my entire life and never meet another Doper f-2-f. I guess it’s just that mama always taught me to make a good impression in every social setting I put mahself intuh. (<----hey, look I wrote in vernacular! It pained me a little, but was at the same time somewhat liberating. Maybe there’s hope for me yet.) 
Happy