Safari extensions from the official extensions gallery will, from Safari 9.0, be signed by Apple:

Secure Extensions Distribution introduces improved security for Safari on OS X. All extensions in the Safari Extensions Gallery are now hosted and signed by Apple. Users can trust that the Safari Extension they install is the one you submitted.

This means that extension developers will have to enroll in the $99 Apple Developer program if they want to update their extensions and, presumably, keep them in the official gallery. Naturally, some developers are pissed off by this, forgetting that this means better security for Safari users. I think this is a good thing.