Right now I probably have the world's fastest JSON parser. It uses SSE instructions and bitslicing and is ridiculously fast - on the order of strlen speed. Of course, it's totally ridiculous but it was fun to write.
I was thinking about binary serializing JSON (I definitely don't like BSON). Simply tagging strings with length and not bothering about escaping helps greatly and simplifies parsing greatly. It also can be easily plugged into existing parsers. Type-specific tagging (int4, int8, bigint, float, string, map) would also be welcomed, but there are some fine points with it.