As you may know i'm trying to focus your GREAT plugin into a marketing point of view (url variables tracking, ...)
One of my usual "problems" is to determine wether a form is optimized and understandable by the user or not. In this point, it would be great to have a tracking of all errors occured when submitting forms (no name selected/no name+phone...)
That's very specific and I believe it won't end up in the product, but I'll give it some thought. The problem is, that for Ajax, only Javascript validates the user input, so you can't report back those errors.... (not easily anyway)