I like to think I'm a very accomplished writer, though I'm probably just "pretty good" at this stage. While chatting, posting on forums, etc. I generally try to be more straightforward and conversational, so I leave out the complex diction and flourish and the most part. It can just sound pompous when you're in a casual setting.

I never learned all the grammatical terms for sentence structure and phrasing, which is bad. But I naturally form sentences correctly, so it's not very often that I have to worry about comma splices or other grievous errors. Also, I always double space after a sentence. It bothers me when people don't.