A facile answer to this question would be to say, "It's the opposite to WhatIsAGoodPost," which you should also read, of course.
MattHaughey comments briefly about one [here], another example can be found [here].
Some interesting points: bad posts can generate a lot of discussion and comments. Good posts can have almost no comments. The moral of the story is that you shouldn't judge whether your (or any) post is good or bad based on the number of comments. Likewise, the quality of comments is often independent of the quality of post; a bad post can be rescued by good comments, and a good post can be ruined by bad comments. That's what life is like in an online community.
Many times, newcomers will be rebuked around for linking to websites that are assumed everybody reads, are old hat, or cheap knock-offs of MeFi and not considered part of WhatIsAGoodPost. The sites that seem to have this effect are: [The Onion], [Salon], [Slashdot], [Plastic], [Goatse], [Fark] and [Wired], among others.
Specifically, posting a link to goatse or any similar site will get you banned. Posting sites that are not safe for work should be prefaced with a "NSFW". If you post something that is not safe for work without this tag, you will get complaints and may be suspended or banned. People read this site from work so please respect that.
Likewise, there are topics that are regularly discussed (and some might say beaten to death) on MetaFilter. This doesn't mean that you shouldn't post stories about them if you feel that the linked page is particularly interesting and adds something new, but it does mean that if it's yet another opinion piece that follows a recent post on the same topic, you should think hard about whether it's suitable. These topics include:
and more
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