Before I get to far into a topic I'm not an expert in, I'll refer you to these:

http://matasano.com/articles/javascript-cryptography/

http://tonyarcieri.com/whats-wrong-with-webcrypto

I'd say the main issue with that is sending an encryption package over an insecure channel. While you could verify the package independently (check a hash against a publicly known one) you're already into advanced user territory and the "encryption for everyone" point of this is gone.