There's an interesting problem that is still unsolved in this space. Most people want JS crypto so that webapps can provide "end to end" encryption, relieving participants from having to trust the webapp service with their plaintext. The problem is that trusting you get the correct JS from the webapp for every message you'd like to send is reducible to trusting the webapp with your plaintext.
I think there are probably some novel solutions to this problem, but it would require work to be done by the browsers.
We seem to be on a track now such that in 5 years, all the mainstream browsers will support a set of crypto primitives built into the browser, implemented in C, and exposed via Javascript bindings. And that's fine.
But it means that we're not going to have end-to-end crypto in web applications for another ten years, because these schemes don't degrade gracefully; the majority of users will have to upgrade before they become useful.
And I'm not sure that this would be enough, either. Crypto primitives are nice, but you still have to trust the webapp to deliver JS which calls those primitives every time you visit the webapp. I believe that this is again reducible to trusting the webapp with your plaintext, since the webapp could selectively intercept any message it chooses.
A bigger related problem: the argument that browser crypto extensions should be high-level (autogenerating secure keys, automatically MAC'ing ciphertexts, picking the block cipher mode for you) is not winning the day. So what we're going to end up with is an OpenSSL-style interface:
x = new Cipher("AES");
x.blockmode = "CBC";
// defaults to all zeroes
// x.iv = "ABCDABCDABCDABCD";
x.key = "YELLOW SUBMARINE";
x.encrypt << "This is my plaintext";
In a sense, this is great news for me, because it means I get another 5-10 years of crypto vulnerabilities to get paid to eradicate. But if you're hoping for simple, usable cryptographically strong security in common web apps... well, keep hoping.
I think there are probably some novel solutions to this problem, but it would require work to be done by the browsers.