I've added all the new things that have come in 0.9 to the changelog
and organized it a bit, so for those wondering what all the new things
are, this may be a good review:
https://github.com/ansible/ansible/blob/devel/CHANGELOG.md
TLDR:
* everything is faster
* lots of module improvements and modules
* user module now supports BSD/Solaris, as well as service module
upgrades for BSD, means things are much more nicely cross platform
* when and 'group_by' allow compelling alternatives to the somewhat
complex 'only_if' construct *
* a gazillion new types of plugins, all about getting data into your
playbook in interesting ways, if you need that, but many won't
All the new things are not on the doc site yet, unless they are module
related, in which case, they are and are flagged for what is new in
0.9.
I need to write some sections on the new constructs, particularly the
new "when_" and "with_" operators, and the other ways of getting data
from lookup plugins.
There are so many types of plugins now it's starting to confuse *me*,
which is a good sign we need to stop making more types of them 
Thankfully, I think we are very
close to mission accomplished around the core, and most future
activity will be around modules. This is good!
On the developer docs page, I'm also going to be adding info about all
the other new plugin types.
Whew. That's a lot of change just for 0.9, and that's *WITH* cutting
some things out of it!
BTW, the next release will be numbered 1.0. This is just a number.
Don't read anything stable or unstable into it. There is no major
architecture shift, and we've kept
backwards compatibility very strongly for the last 5 or 6 releases,
I'm not planning on changing anything now either.
--Michael