Have you considered Xcode?
I know you said you don't like TextMate, but it's one that meet pretty much all your needs.
which is fast
TextMate
Really fast
Xcode
Not that fast but not Eclipse/Netbeans-slow
which does syntax highlighting
TextMate
Very complete and extensible
Xcode
Does syntax highlighting, it depends on your needs
were I can define own syntaxes (or there are many plugins e.g. for YAML),
TextMate
Very, very extensible
Xcode
Syntax extensions are possible
where the developers are active,
TextMate
Some say it's vaporware, yet very few other text editor can compete with its current features
Xcode
Apple development of this program is very active
which does not have the look & feeling of TextWrangler
TextMate
If you don't like the sliding bar, there's a plugin to replace it by a drawer
Xcode
Doesn't have a sliding bar
which can define projects (like Coda can)
TextMate / Xcode
Do projects
which includes a basic support for Git and/or SVN
TextMate
Support for Git/SVN/Mercurial (and probaby more) via built-in and added plugins
Xcode
SVN support for 3.x, added Git in Xcode 4
which supports a fast autocompletion
TextMate
Basic variable/function completion
very complete code completion via bundles
Xcode
Basic variable/function completion
MacOS X 10.6 compatible
TextMate
Some compatibility issue
Xcode
Fully compatible
I haven't been experimenting with this for long enough to be an expert yet, but I think you're confusing two semi-separate features: autosave and versions.
Autosave saves frequently -- in TextEdit, it seems to save after about 30 seconds of inactivity. But it doesn't save a new version, it just saves the current document state.
Every hour and/or every time you explicitly save a version (Command-S), it saves a restorable version -- one that you can revert to if you decide you've made a mistake in editing. Once saved, the "version" does not get updated; you can revert to it, or go into the Browse all Versions interface and copy content from it, but it's essentially frozen.
Autosave protects you against forgetting to save changes. Versions protects you against mistaken edits you wish you hadn't made. They operate on different time scales (and as far as I know, neither has customizable timing).
Best Answer
TexShop, the Latex editor, now supports Lion versions. It is not a general Text Editor but can be used as such and supports many of the features you requested.
http://pages.uoregon.edu/koch/texshop/lion.html