So far, I'm impressed. The test libraries and module system are quite slick, and the infrastructure for uploading new modules is starting to fall into place. Work remains to be done, but it's clear that progress has been made and they're on the right track.
After playing around with this stuff for a day or so I'd say there are a few places that still need some work.
First, there's no tool for automatically installing the packages, so you are basically stuck just downloading the tarball and copying the .js files into place manually. This isn't a huge problem, but it means that dependencies are kind of a pain in the ass.
The dependency problem rears its ugly head most noticably in the testing world. If I want to use the test libraries I basically need to bundle them with my module, and for the life of me I can't figure out how to make use of them via manual inclusion of the .js files, so that means I'm also stuck bundling a version of JSAN.js.
Finally, and this is only going to affect people who are actually writing and releasing modules on JSAN, the upload server (JAUSE) doesn't give you much visibility into what's going on. I uploaded my first module last night (well, really early this morning technically), and it hasn't shown up yet, and I have no way to tell if everything worked or not. In CPAN land the upload server seems to give more feedback to the user, via email and the ability to look at log files for the PAUSE daemon.