Popular subjects that Urban Dictionary users like to cry over:
-George W. Bush, and how he's personally oppressing you.
-Anyone with any mainstream religious beliefs
-Anyone who gets laid on a regular basis
-Television and the media
-The "rap" music
-Video game consoles
-Anyone who dares have a different opinion on a subjective subject than you.
-Anyone who doesn't share the typical affluent angsty white male middle schooler opinion
The light side is, if there is a widely used word, that's not in the English vocabulary (officially), and you want to know what it means, chances are, you may find it here.
Now, for the dark side: poor monitoring. Though you may find the word you're looking for, chances of finding it on a definition that is accurate, non-opinion-showing, and un-hateful are extremely rare.
Because of how poorly monitored the site is, Spammers take advantage of it and post words with definitions that are inaccurate, hateful, violent, offensive, and/or maybe even porn-related.
Fortunately, there is the "recommend for removal" feature and sensible people can come and have such definitions removed. However, the Spammers can post as fast as (if not faster than) those people can remove.
One definition you might find could be as bad as the example below:
(Please note that I have nothing against the television show "Barney". That is just an example.)
Also the place where good people become corrupt.
I became corrupt after clicking random several times on Urban Dictionary!
Although Wikipedia is also editible by anyone, Wikipedia is a much better site to look for definitions. It's more of an encyclopedia, but their community is alot better than this site.