Use JSON for data transport in AJAX calls
We are currently using a homegrown tab-separated text format which comes with all the problems related to handling special characters, etc. Ok... most of this has been fixed, but it is still a bit cumbersome to generate and parse.
JSON is becoming a de-facto standard in web applications. There is plenty of existing libraries to use on the server-side and newer browsers have built-in support for parsing it. The format as such is also a lot more flexible than our homegrown format.