Thanks, will fix momentarily! EDIT: fixed, thanks!

> How many dimensions are supported?

Two dimensions. The commands are designed primarily with Earth geometry in mind to help people build location-aware apps.

> Also, does this support many projections, or just a few.

It supports WGS84 (a commonly used ellipsoid model) or a unit sphere. Currently you can't use a plane. Check out http://rethinkdb.com/api/javascript/get_nearest/ for an example.

> Does this do a correct interpolation of degrees to meters along the path

The implementation is based on the S2 library (https://code.google.com/p/s2-geometry-library/) also used by PostGIS, which AFAIK does spherical linear interpolation.

> Are there commands to export WKT or WKB?

No, currently only GeoJSON (which you could later post-process and export to other formats with existing tools). This is a great idea, though, if there is more demand for it we'll definitely add other exporters.

EDIT: all great questions, I opened an issue in the doc repo (https://github.com/rethinkdb/docs/issues/521) -- we'll address all of these in the docs.