Oh, loads. I used to write up all kinds of posts, some of them long and involved, and then chicken out and not post because I figured what I had to say wasn’t important enough.
But nowadays, I usually move on even before typing them up.
I seem to go through cycles of activity and lurkerdom, but always a certain fraction of my posts fail to make it through my phone line.
I do it pretty often. I usually type out a reply, then look at it and see it’s not adding anything useful. Or I start to open a thread and realize it’s stupid.
Sometimes my reply is something I might say around close friends, because they know where I’m coming from, but strangers might interpret it differently. It’s also very difficult to tell “tone of voice” in a posting. So if you are just kidding, you better say so.
Um, pretty much every time I’ve wanted to post, I’ve changed my mind. So, hundreds of times, in my case. shrugs
Upon closer reading:
Holy crap, SpasmicChaos, are you me?!
I usually self-censor before I even bring up a reply window. If I have nothing to add, I don’t add it. Except in Rue’s MMPs - the whole point of that thread is to add your own personal nothings.
There have been a few dozen times when I changed my mind after typing out a response, or when the boards were wonky, if I couldn’t get it to post after a couple of tries, I usually decided what I wanted to saw didn’t merit the frustration.
I try to avoid “Me, too” posts, although I’ll post condolences or congratulations rather freely.
The ones I don’t normally submit are the ‘yeah, and me.’ posts*. For example, I was going to post a reply to the thread in CS about Underworld saying something along the lines of ‘Yeah I liked it, it was a no-brainer film with a pretty lady in leather, etc. etc.’ but I chose not to because it didn’t really add anything to the thread (loads of people had already replied saying what I was going to), and I feel guilty about taking up bandwidth needlessly.
*apart from this one.
(looks at post count)
Not often enough, apparently. I like to think my contributions increase the signal-to-noise ratio, rather than the reverse. But some days I’m not so sure I’m managing that.
As you may well know, Lola, I don’t stop myself NEARLY often enough… 
Probably a third to half of my posts that get written don’t get posted. Not the mention all the times that I’d like to answer, but decide that I should probably keep my mouth shut, and don’t write anything at all.
Yes, but when my personal nothings are far below the quality of other’s personal nothings, I don’t add on. My personal Muse tends to take an extended coffee break when I really could use its advice and inspiration in replying to a post.
Some days I feel like I could make Joyce’s Ulysses look like a Reader’s Digest excerpt. When hand goes to keyboard, the cranial-to-brachial-to-digital connection craps out somewhere. And the Muse is brewing decaf.
Bummer.
Quite often, by the time the reply thing finally appears I’ve forgotten what I was going to post. Then on other ocassions I decide I can’t be bothered to post …
Less than I should.
Ditto, at least recently. I deleted quite a few lengthy posts addressing the SF’ers, partly because I didn’t feel like mixing it up with them, partly because I thought it somewhat of a wasted effort ( the possible edification of lurkers aside ), and partly because others were doing a reasonable job and my input wasn’t necessary, strictly speaking.
I also deleted a number of ME-related posts, for semi-similar reasons ( arguments are often tiresome and repeititive, plus the partisan rhetoric on both sides annoys me ).
More than I used to, less often than I should. I sometimes find, while typing a reply, that I know less than I thought I did, and I realize that what seems a sharp correction may actually be a bold step off the edge of a cliff. I delete those, when I spot 'em.
I stop myself a lot… Half the time, I just lose interest in what I was saying, so I figure if I’m bored with it, no one else wants to read it either. Or it’s a classic case “That sounded so much better in my head”… I see something on the screen and realize… Eh, that’s not really worth posting.
I do it a lot. That’s why my post count is so low.
Occasionally, but more often than not I’ll let the post go through only to wish I hadn’t hit that button. Nothing I write is world shattering and I often feel totally out of my depth among such witty and knowledgeable people. 
If I his “Submit” after every post I’d typed up, I’d probably have a handy-sized post count. And similar posting priviledges, to boot.
Absolutely all the time. Interesting how a lot of people have replied to this thread…