Clark Realty has told D.C. that they will no longer develop Poplar Point. Greater Greater Washington has a good post on part of the proposal for Poplar Point's development, namely, a massive park. They make the point that a massive park is really nothing more than an invitation for crime and a waste of space. Obviously Central Park is an example of the contrary but I do tend to agree with the basic thrust of their argument (and arguably Central Park's success is a function of Manhattan's tremendous density and policing techniques). San Francisco is a city that does seem to strike a nice balance between green space and city space and they achieve this through constant distribution of small green spaces. A really cool way of implementing this at Poplar Park would be to do the Urban beach thing. Anyhow, my two cents.
Update: Another exception might be the Tiergarten in Berlin.