The main complaint I have about Urban Dictionary is that many of the entries seem to focus on sex positions and other sexual matters, sometimes in cases where a more child-appropriate definition should exist (see e.g. the entry on "Strawberry Shortcake"). Also, I have doubts as to whether the site is able to adequately prevent cyberbullying (though I have no evidence available to indicate a specific problem). Other than that, I find Urban Dictionary quite useful.
Urban Dictionary is a slang dictionary with your definitions. Define your world