In the UK waste ground would commonly refer to what I think you guys in the States would call an empty lot - or similar. The general connotation I would understand is a patch of ground, not neccessarily extensive, which is wild and undeveloped by contrast with the surrounding townscape. It is also virtually axiomatic that such an area makes a *fine imaginative place for youngsters to play...!

I think TS Eliot clearly drew on both his USA and UK language experience - never more so than in his most famous poem...