Super-long incubation period from "rc2" to "release" tag did not, of course, result in any new features, because of a feature-freeze. We did add a few missing calls and features to APIs and SphinxSE and also a couple of optimizations but that's catching up, not really adding new stuff. On the other hand, looking back at 40+ bugs fixed in this release, I would still say that rc2 was pretty stable, as most of the bugs were either minor, or rare, or both.
Most importantly, index merging is now much more stable. Interesting that it's been around for quite a while, but many bug reports were only filed against 0.9.9 or even 0.9.10 (and then ported back). Perhaps most of you are only adopting it now. So note that 0.9.9-release is an officially recommended occasion to re-evaluate merging if you were having issues with it before.
Another major change worth a mention on the front page isn't directly related to the code, though. Old news to those dedicated people who follow our commits on Googlecode or #sphinxsearch on Twitter, and new news to everyone else, Sphinx now has official IANA assigned ports, 9312 for native API and 9306 for SphinxQL, so update your sphinx.conf and /etc/services. (On a side note, does anyone know what is this appman-server thing that owns port 3312? I'm still occasionally curious about that.)