Overly derivitive characters really destroy role playing, mainly because they're hard to take seriously in-campaign. Especially when they're derivative of like three or four characters and are rather shameless about it.
For instance, I had someone in my last WoD game that wanted to play a hunter who was the grand daughter of Van Helsing that had Hellboy's gun (complete with special ammunition) and was part of this Buffy the Vampire Slayer like prophecy.
The Van Helsing thing I coulda worked with. The prophecy even I could have worked with. But she wanted to tie all three together. In the end, I let her go with the Van Helsing concept, told her to just use a heavy pistol and that there may or may not be some prophecy about her but she wouldn't know.
However, she (her character) was completely insane. Started blabbering all this stuff about vampires and pulling out high-caliber handguns at completely inappropriate times (the other PCs knew nothing..or very little...about the occult and certainly had never met any real vampires). She came pretty close to getting arrested by the SWAT officer that was in the group more than once; and would have gotten arrested had the situation not been what it was.
And then there was the guy that wanted to play the detective named Gabriel Knight....