"Density" is a favorite among almost anyone who thinks about cities. Most people agree that it's a "good thing" and also that there is not enough of it in a world of "sprawl".
I have often blogged about all this with the view that (1) yes, it's a "good thing", but often defined much too vaguely; city-wide average density measures are not adequate; (2) there are enclaves of creative activity in high-density Manhattan as well as in low-density Silicon Valley -- and many other places like it; (3) as the 2010 census data become available, we see continued suburbanization; (4) there is enough suburbanization around the world, to cause us to doubt that it is an artifact of peculiar U.S. policies.
Today's WSJ includes their annual list of "Top 50 Start-Ups". Precise addresses are not given, but I count eight in "San Francisco" (am guessing the city because start-ups located in its suburbs are labeled by their suburban locations) and I count three in "New York" (am assuming Manhattan because another firm on the list is placed in "Brooklyn"). Thirty-nine of the top-fifty start-ups are in places decidely less dense.