Emacs development began in 1976 and has been active development ever since. It was built for extensibility, and for thirty years, users have been adding and smoothing the features that make their daily work easier. The documentation for GNU Emacs is also amazing. As well as the comprehensive manual, there is convenient per-function and per-variable information available from within Emacs.
When I moved from ViM to Emacs in 2002, I was amazed that most of the mildly complex tricks that I learned with ViM were coded into Emacs and available at a key press. It’s definitely the piece of software the contributes most to the efficiency of my computer usage.
Version 22 doesn’t have a killer feature, there are just jumper loads of changes that make things more like you’d expect or more convenient, such as:
'C-x eee'now executes the last macro 3 times, so you don’t have to
'C-x e C-x e C-x e'
A list of notable changes can be seen in the NEWS file. There are new features for programmers, but I’m not one (any more). And there is better support for editing files in Asian writing systems, and for users of non-standard hardware and operating systems, but I can’t comment on any of those features either.
Tips and tricks for Emacs can be found on this independent site: http://www.emacswiki.org/, and the Emacs article on Wikipedia is quite good. And discussion of the release can be found on LWN.net, Slashdot, OSNews, etc.