First off, this is kind of a pain:

myForm.set('send', {url: 'contact.php', method: 'get'});
myForm.send(); //Sends the form.

When in earlier versions of mootools you could just do myform.send({url:'/', method:'get'});

Also, there's no way to specify a form to use Request.JSON which would be super helpful. Maybe make form.send take a Request object

myform.send(new Request.JSON({ ... });

Frankly, the docs on this are very hard to find (who would know to look under the Request object for form submission?) and the form search is useless for this case. It would be great if you showed how to add events as an example to form sends, like onComplete. Because documenting this:

el.get('send')

makes it seem like you can do this:

el.set('send', {url: '/'});
el.get('send').addEvents({
'success': function() {}
})

Which doesn't work. The whole thing is poorly documented, lacking in examples, not straightfoward, and hard to find. I think there are lots of opportunities for improvement here.