For the most part I'm pretty passive online, I dont usually post a lot in discussion forums even though I read huge amounts of them. This also means I don't generally get involved in flamewars at all. Normally I see a comment or post I find interesting and consider writing a reply (like this one), I then start typing a short reply (like I'm doing now) which vaguely expresses my thoughts, I normally write, revise, edit, read, reread re-edit the post several times before hitting submit (like I am doing with this comment). Once I get to this point there is a 90% chance I will say 'screw it', delete the post without submitting and move on. (If you are reading this then you know it was one of the lucky survivors that made it.) In real life I'm pretty laid back so perhaps that explains my ghostly online persona.
You've described my MO as of late pretty succinctly here. I find that writing a comment, with a series of edits, helps me to understand how I personally feel about a particular topic. Once I've ironed out the creases of an idea in my mind, it's less important to me that other people hear it.
That's funny, I go exactly the opposite way. I write out a response, and if it's nice and clear I post it because it feels like an idea worth sharing. Better clarified the idea, the more I feel justified in sharing it. Every once in a while, I'll type out a long-winded response, realize 75 percent of the way through that it's either inane or just based on some huge fallacy I didn't see before typing, and then delete it. Sometimes those are my favorite responses, come to think of it, because they end up re-shaping some basic assumption I'd held for a long time. But yeah, the ones I discard are the ones I don't think are worth sharing.