When I’m commenting on a post I tend to use very high-register vocabulary, to the point of sounding archaic or academic. I think it’s primarily a hold-over from writing essays in school, but it doesn’t hurt that the places on the Internet where I hang out tend to be extremely nerdy.
I also frequently leave my sentences unfinished when talking IRL. I’ll just sort of trail off once I can see that I’ve gotten my point across instead of bothering with a complete sentence.