Is there a free, lightweight text editor that isn't text-only but also isn't a heavyweight IDE?
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 &amp; 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