there's a difference between Urban setting DH runs, depending on if your city has lines for it... but comparing it to real street riding, all you are doing is fighting knobbies and pogoing.... just like Will mentioned... Urban hucking is another creature as well, but those are mostly one hit wonders not worth lugging the bike for.
please search, there was a sticky on this subject when the forum first got it's feet off the ground, and it was possibly one of the most horrible threads ever.... but, if you're looking for something talking about it, there you go.