1) Must have some sort of queue-like behavior, or race conditions. In other words I want a guarantee that all saves happen in order (imagine a checkbox, and due to latency in network traffic of the user checks it on, then off. What state was saved the on or off for one of them? I don't like these race conditions) this can happen for any input type.
2) What happens if the data must be extracted weirdly, or weird parameters sent to the url. A data extractor is a useful concept to have be passed in.
3) Indication to the user that it works. Like what gmail does. A useful feature.
4) What if its not a form, or different elements go to different urls? Also a useful thing to do.
This sort of tool became a cornerstone of some very cool form interaction mechanisms.
I guess the idea of this plugin would be to have a gracefully-degrading auto-saving form that preserves values while the user types for recovery purposes.