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Apparently, you need some reputation even to leave a comment to answers to your own questions. That seems unnecessarily strict.
It's not correct. You can always leave comments on your own posts and on answers to your questions.
Ah, thanks for clearing that up. I suspected as much, but couldn't think of any way to test it without access to the source code.
I think this came up somewhere else, but I figure I should probably mention it here too. Some people may ask, "why can you leave answers with no reputation, but need 50 reputation to comment?" This leads to situations where a new user wants to leave a comment, but can't, so she has to leave an answer, mildly annoying everybody. The reason you need reputation to comment is to prevent spammy comments. Since new answers bump the question to the top of the homepage, it's much harder to sneak in a spam answer (without everybody flagging it as spam) than it is to sneak in a spam comment.
I assume you are talking about the Wedderburn question? Answers you receive does not factor into your rep.
You get +10 for each up-vote and -2 for each down-vote you personally receive on your questions and answer you provide (not the answers others provided for you). (This is, by the way, already explained in the FAQ.)
The change from 21 to 25 rep is most likely that one person voted you up for the question, and three people voted you down, giving a net of +10 + 3 * (-2) = +4 of reputation, and +1 + 3 * (-1) = -2 of votes.
Indeed, if you click on the "reputation" tab in your user profile, you will see that you have +10 and -6 (so net +4) reputation for the Wedderburn question, as WillieWong suggests.
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