This is really quite an impressive idea.
I particularly like the way Functions are implemented. It seems it should be possible to implement many of the APL functions that I found so powerful, in this environment. Since the APL primitives are well defined, those definitions may well provide a bit of a head-start. I also really like the idea of being able to read values from web pages since this is quite a bit easier and more general than parsing the underlying HTML.
I'm curious as to whether you think it might be possible to define a set of functions and then pass them the values as URL arguments. If so, I could easily see using AJAX and JS to "submit" computation requests, perhaps even within an iFrame. In effect this would create a form of embedded, programmable calculator.
In short, it's already an intriguing concept. With luck and a bit of effort from other contributors, I think it has the potential to become a very useful service.
Cheers,
Hans Wobbe