My biggest problem with SASS is that it's turned into a full on programming language.
Incidentally, it's also not Turing-complete.
Wikipedia: After its initial versions, Nathan Weizenbaum and Chris Eppstein have continued to extend Sass with SassScript, a Turing Complete scripting language used in Sass files.
and http://wiseheartdesign.com/articles/2010/01/18/the-demise-of...
And I'm the guy who invented most of the programming facilities of Sass and I'll tell you, they're not very good programming tools -- but they're enough to make some really great stylesheets.