Following a pertinent link from that discussion: A series of psychological studies posits that people who are really, really bad at something don't know it, and they're bad at that thing precisely because they don't know it. And one of the things they tested was ability to recognize humor:
"To assess joke quality, we contacted several professional comedians via electronic mail and asked them to rate each joke on a scale ranging from 1 ( not at all funny ) to 11 ( very funny ). Eight comedians responded to our request (names removed by me, for reasons you'll understand in a moment. I didn't recognize any of the names, if you're wondering how famous [or not] they are.). Although the ratings provided by the eight comedians were moderately reliable ( a = .72), an analysis of interrater correlations found that one (and only one) comedian's ratings failed to correlate positively with the others (mean r = - .09). We thus excluded this comedian's ratings in our calculation of the humor value of each joke, yielding a final a of .76."
So basically, what they're saying is... one of those comedians turned out to be unable to recognize humor in a way that correlated to the rest of the group. One of those comedians... isn't funny. Oh dear.
Anyway. The conclusion of that part of the study? People who are really, really unfunny are really, really unaware of it. Also: You ever wondered why so many awful, awful singers take it upon themselves to try out for American Idol? Now you know. Their awfulness actually correlates to their unawareness of that same awfulness.
Curiously, people who scored in the top range on that joke quality questionnaire actually underrated themselves. I'm not going to leap to a one-for-one conclusion that skilled people underrate themselves to the same extent that unskilled people overrate themselves, but it's sort of like that idea that the more you learn, the more you know that you don't know.
Here's the really scary part: Ask yourself what it is that you think you can do well. Can you really do it well? Do you have objective, material verification of that fact? By which I mean, "All my friends say I'm a good writer" is neither objective nor material. "I've published six books that were critically acclaimed and/or sold a frillion copies each" is an over-the-top example, but it is both objective and material. (We can get into the popularity vs. quality publishing argument later.) Basically, if you're really God's gift to whatever, you'd have gotten somewhere with it by now. Example: Ask me and I will tell you that I am a really good cook and have a great singing voice. I don't so much have any proof of this. Particularly since I don't cook all that much, and I don't let anyone hear me sing. I am very likely overestimating my skills here. But once I realize that, and I know that I need to improve, and I decide that I would like to improve, I can set about cribbing cooking lessons from my sister, or practicing my shower-singing. Or I can just admit that I will never be a four-star chef who sings backup professionally. See?