@littlek,
That sunken plaza seems deadly, Farmerman.. Interesting to hear about the newly popular space between the Renwick and White House..
These squares have different components, sizes, shapes, cultures.. (on and on she goes, bla bla) including making a swell site for a bonfire of vanities, or a nice place for burning heretics. I like a lot of them, from the space djdj showed, to pz. Mattei (god, that fountain makes me cry), and agree with panz on the mixed use thing.
But they can work in their simplest forms if there is something there - a small corner store, a bar in the italian sense of coffee, tea, sweets, a place to tuck into in the rain on the way to catch the bus to work.. a few benches, etc. A basic place to walk
to. My big interest is in what I call 'pedestrian culture'.. and these spaces can help make that happen. Jumps off of soap box - but wait, soap box: that reminds me of Pershing Square in LA. Now there's a continuous flop of a space.