OK, this may sound a bit of an odd one, but I have been struggling with this for a while. See, my definition of urban fantasy is, in a nutshell, Harry Dresden. It's fantasy, in a mundane urban environment, usually where the general populace don't have a clue of what is lurking in the shadows around them. Simon R Green's 'Nightside' books come broadly into this catagory, as do Tim Waggoner's Matt Richter series.
But then along comes the 'new' Urban Fantasy: Buffy! Kim Harrison! A plethora of hard hitting 'girls with attitude and the moves to make it stick'. Mandatory love triangle, usually involving at least one paranormal. The cynic in me says its a highjack job - paranormal romance looking to get respectable by including some rough and tumble :) I'm mostly kidding. Mostly.
I read an interesting blog today. The long and diverse history of urban fantasy, by Carrie Vaughn. She nails the hijack down to around 2007. Dresden started around 2000, Nightside around 2003. I dont know enough about Waggoner's work to say if Richter is his only Urban Fantasy, but that was around 2009, so it sounds about right. Anyhow, her article is well worth looking over.
I've no problem with a genre evolving. Heck, I really like Kim Harrison's work (and love the pun titles), and Buffy works on so many levels for me its unreal. I also love the 'Demon Trappers' trilogy by Jana Oliver. But at the same time, there is a sense of Urban Fantasy as a label being refined down, and excluding much of what it once was in favour of marketing it to an ever more specific slice of readers. That I am not in favour of . Genres are already too specialised, and the boundaries between them are already to high.