Usually, if I'm reading a thread I have interest in, someone has covered most of what I wanted to state anyway.
Sometimes I DO have something relevant and new to add, so I start to post. Then, I'll either take too long to think about how I want to word what I have to say, worry what I had to say was pointless anyway, and either abort the post or pull it back to some kind of one-liner. It probably makes most of what I post pointless to spite me anyway...
If I feel I've made a sentence with too many commas, and can't find another way to word it, I'll usually delete the paragraph it made in an attempt to avoid punctuation errors. I guess I feel like what I say would be somewhat invalidated by them... "Oh, I guess he can't spend the time to think out his sentence construction, I wonder how hard he thought about what he had to say in the first place?" (Holy crap... Am I Obsessive Compulsive?)
I could post up a storm of wonderful and interesting if I ever let me. (At least that's what I tell myself, all the head shrinks tell me I'm afraid of success.)