The easy way to get me to not comment on a weblog
It's simple: just don't offer any sort of comment preview option.
This insures that I will never leave a comment, no matter how much I have something I want to say. Without the chance to preview my work before I'm committed to it, I feel too nervous and it becomes too much of an annoyance.
This is one of those blog software things that really puzzle me. It is not as if blogs all format comments in the same way, so I would think that software authors would see the attraction of people being able to check that they got it right before they post their comment. (For bonus points, a number of the 'no comment preview' blogs I've run across also have no explanation of how your comment text will get formatted, or how to include links, or etc.)
This isn't the only reason to want to see a comment preview, of course. In many cases, comment preview is the first time you see the whole text of your comment at once, since most comment form textareas are dinky and grow scrollbars once you have more than a little bit of text (which make it much harder to get a sense of your comment as a coherent whole, since you have to hold everything out of the scroll area in your mind).
Another effect I've personally experienced is that it's hard to judge how big a paragraph or block of text will be in a comment textarea, because it is in a different font than the final version (textareas are all in monospace, and often in a different size). What feels right in the textarea not infrequently turns out to be too small and too little text when I preview it. (This can be exacerbated by links and the like, which take up extra space in the source but not in the final result.)
(And I just plain benefit from being able to step back a bit and reread my draft comment; often what looked good when I wrote it is not so hot once I preview it. I revise my draft comments a lot.)
While I have a rather limited sample, looking at comment form usage patterns here on WanderingThoughts suggests that I am not alone in this; a number of commentators here seem to go through multiple preview drafts. (Not all of them, though; some are more write and go people.)