I wonder, why introduce (yet) another syntax, not <style type="text/x-sass"> (or text/x-less) support. The latter can be used in supporting browsers natively, while there are existing JS helper libraries to provide support to other browsers.

Using SASS would be introducing Yet Another Syntax. 99% of people who write CSS will never have used or heard of SASS.

Sorry, but this sounds illogical to me. How introducing some additional new CSS syntax is less Yet Another Syntax than just using some already established (but not popular) syntax? SASS has its user share (and easy to learn too) and new syntax has no users at all.

The new syntax has no users at all sure, but CSS has far more users than SASS or LESS will ever have. You're asking web developers and browser developers to learn and support a completely different technology, rather than adding an incremental, much needed improvement to something that has been used for many years.

Mixins seam the be the most useful feature to me, the other two seem more like syntactic sugar to me.

Strikes me as over complicated. Add variables to CSS. Done.