There are loads of things, but one trap I used to fall in was that many browsers accepts JSON without quoted names, while ie6 and ie7 does not.
{ name: "Jakob" } // will often work, but not in ie6/ie7
{ "name": "Jakob" } // Better!
Edit: To clarify, this is only an issue when actual JSON is required, as opposed to an object literal. JSON is a subset of the object literal syntax and is meant as a data exchange format (like XML) which is why it's designed to be pickier.
