The unit in which the distance is returned is not clearly specified. From my understanding, it depends on either the index type or the data type. Somebody asked about on IRC a few minutes ago, I had to check out some old code to guess what was returned. The doc did not help much.

As a side note, I think the 2d vs 2dsphere and legacy pairs vs geojson data differences of behavior are highly confusing. They should be systematically pointed out in every "reference" page, on every parameter or return value they are relevant for.