> At some point, you could perhaps mash up data you get about buildings with a lot of the data NYC has opened up about the city as part of its gov2.0 efforts. This way, you can provide a sense for the "value" of an apartment alongside stats like average income of neighborhood, crime rates, etc.

Already got some heavy competition in that area: http://nyc.everyblock.com/ http://streeteasy.com/nyc http://www.trulia.com/