Is there really need for every language to have its own web stack? Most do work with FastCGI, but treat it as second class citizen (or so it seems as an outside spectator). This is somewhat similar effect to that every language seems to have its own package manager.
I can understand why some newer, better protocol than CGI or even FastCGI might be useful. But why there isn't a "FastCGI 2.0" which would have the advantages that these language-specific middleware boast, but which would be language-agnostic on the server side. At least from my point of view most languages are not that much different that they'd couldn't fit in a common interface.
Well, by analogy with Rack, WSGI, Ring, Plack, etc, widget systems and forms and AJAX are way outside the scope of the problem that Clack is trying to solve.
All of these systems boil down to providing a simple convention for web applications to interface with web servers by implementing exactly one function that takes the environment and returns a triple of response code, content-type, and content.
Everything else you describe would be built on top of this, not built into this.
* WSGI (Python) - http://wsgi.org/wsgi/
* Rack (Ruby) - http://rack.rubyforge.org/
* Plack/PSGI (Perl) - http://plackperl.org/
* Ring (Clojure) - https://github.com/mmcgrana/ring
* WSAPI (Lua) - http://keplerproject.github.com/wsapi/