Jack Betts outlines the political (i.e., non-market) measures to "save urban spaces." Under the ambiguous rubric of the "common good", vocal interest groups willing to picket and put up signs in their yards are hardly willing to come up with the investment capital needed to match their stated preferences for the property. (Read: We won’t buy it, but we’re willing to apply state coercion to get what we want.)
That land go to its highest and best use is something that only a real estate market can determine. Moreover, efforts to conserve urban spaces actually results in more sprawl. Such efforts are thus totally at odds with the usual, eh hem, "smart growth" aspirations. All this goes to show that planning doesn’t work, that people have conflicting and contradictory values, and that markets do the best job of reconciling those diverse values. People who lay claim to the "common good" are justifying expropriation.
(I argued in another post that Dix, for example, should be auctioned.)