As reading through the comments shows, too many Americans think "city living" = tall apartment buildings. It is true that urban living and too many detached single family homes aren't all that compatible. But really what ruins density - density is good for some reasons, bad for others - in cities are required setbacks, mandatory parking, minimum lot sizes, too many surface parking lots scattered around, etc. Put too much space between people and places and suddenly it's harder to walk anywhere.
Much of Philly is 2-3 story row houses on small lots. The lots are small. The two story row houses aren't huge, generally, I'll admit, and perhaps not large enough for what people generally consider to be big enough to raise a family in these days. The 3 story ones can be quite large. Not McMansion large, but certainly large enough. Want a nice big back yard? Those are a bit harder to find, and new construction/rehabs tend to gobble them up in order to build more indoor space, but they exist. Again, mostly not suburban big back yard, but big enough.
Yes I have shared walls. It's old solid construction. I almost never hear my neighbors through the walls. I hear them on the back porch sometimes, but I'd hear a party my neighbors were having in the suburbs, too.
If I ran the zoo I would do more upzoning to allow taller buildings in certain places. Not in all places, just along certain commercial corridors and along major transit lines (especially around stations). But the city's going to be mostly 3 story rowhouses for a very long time, no matter how much it booms (and it isn't booming, just very slowly increasing population).
