The key discovery part seems unobjectionable, and vastly cleaner than what keybase.io does.

Looking at their design for key sync[0], though, maybe I'm just dense, but I swear I read the article, and I still can't tell -- what's the advantage of this complicated thing with a symmetric key over just protecting the private key with a strong passphrase and sticking it in Dropbox?

[0]: https://blog.whiteout.io/2014/07/07/secure-pgp-key-sync-a-pr...

(Okay, maybe the advantage is that you don't have to rely on the user to choose a strong passphrase.)