Mongo's probably still got the edge as a JSON store overall, but definitely check out the new JSON object dereferencing functionality coming in 9.3. There's a Russian indexing posse consisting of Oleg, Teodor, and Sasha who have been looking at doing proper indexes for JSON but haven't managed to secure funding. (Disclosure: I think they should get funded.)

These are the same guys who built hstore, full text search, GIN and GIST indexes and I think are working on a generic regular expression index type right now.