Why is state of NJ so much greener than the rest of the map

What appears on the default Mapnik map is a compromise - it does show "every clump of trees" (or every natural=wood at any rate). The reason why NJ's so much greener than neighbouring states is that woodland data appears to have been imported for it by this account. Looking at the map N of Trenton it seems to include "land that is sometimes used for growing trees" rather than "land that has trees on it as at the time of the Bing imagery".

Other renderings are available - you may find that this one fits better with leftpondian expectations of what a map shoould look like.

The data is valid and appropriate to OSM, all the way down to individual trees on private property - if the mapper has determined that the data is correct and current.

The New Jersey land use data has been mass imported from other public data sources. It's my opinion that such a mass import does not add value to the OSM data:

1. OSM editors such as Potlatch and JOSM do not separate the landuse data at edit time, thus massively complicating future edits.
2. Mappers who improve or create landuse polygons stand to lose their work if newer data is imported.
3. Public land use data would be better stored in a separate database, and combined with OSM data at render time to create colored maps - if needed by the desired render style.