JSON (for the most part) is a nice format to work with, aside from loosely defined datetimes as mention.

The two areas where I believe the format can greatly be improved; 1# having a standard to define the structure (sometimes schemas can be handy!); 2# a stranded binary format, yes right now with have UBJSON (which doesn't have a date format, this is worse in binary) and BSON (which contains some MongoDB specific stuff).

I'm not saying they don't have their place, but.. Protocol Buffers are more akeen to .net or Java serialization, in the they're quite fragile if used with different versions and/ or with different vendors.