I'm 50-50 on SF. I did grow up in SF (west of twin peaks, though, so a less urban experience even by SF standards), but I was exposed to a more urban experience when I lived for a year in Paris when I was a kid, and later NYC for a year as a young adult (in my 20s).
SF really is pretty diverse, especially in terms of language diversity (I read somewhere that the number of languages spoken by 1000 or more households is higher in SF than even NY, though I don't have a cite for this). SF also does have decent density by US standards as long as you're not talking about NY. (http://beyonddc.com/?p=4808). I also find that getting out of the city for the countryside or natural surroundings is faster and easier in SF than almost anywhere else (ie. the time it takes to get from a very high density urban neighborhood into a very quiet forest or rugged, undeveloped coast is amazingly short in SF). Since I really value both things, I do get a lot out of living in SF, but there is a kind of truly urban neighborhood, with trees and open pedestrian areas, that is much more prevalent in larger, more dense cities like NY or Paris. The images in that link about curb cuts, contrasting the street scape between Park Slope and Dolores Street in SF, pretty much sums it up. The image of Park Slope manages to both peaceful and highly urban at the same time, largely because it's urban enough to give up on cars and driveways. It almost seems like SF got just close enough and then blinked.
Aside from NY, though, and maybe a very few parts of Boston or Chicago, there really isn't much else out there, at least in the US. For the west coast, you just aren't going to find as many of those little red dots clustered (high density), even in Seattle. Interestingly, LA is becoming much more urban. That said, SF's high density urban neighborhoods are pretty limited in size, and they are breathtakingly expensive.