An interesting discussion:
http://www.sfu.ca/~finley/discussion.html
Excerpt:
"I recently talked to a person who admitted that for many years, he thought there was a rule that limericks had to be about sex. Well of course, this is not true, although it helps. My opinion is that noble sentiments, world peace, a nice image, small town nostalgia, sincere self-pity, or the like are unsuitable for a limericks. A good limerick must have some element of the absurd.
The most obvious path to absurdity is to be bawdy. Here one has access not only to the risque theme, but also the host of euphemisms, which become weapons in the limerick writer's arsenal. If not bawdy, limericks should still be rough in some way, such as through ludicrous use of language, ludicrous situation or ironic comment. Of course, if more than one of the following can be combined, the limerick may be better."