@Jason Resnick - silent extends, for loops, conditional statements, sass syntax, dynamic code generation... In my opinion, SASS is one step closer to a scripting language than LESS is, and more familiar to both PHP programmers (in syntax) and those who don't want to get into a preprocessor (with SCSS syntax).
There is also no equivalent to COMPASS in LESS. Just like WP pretty much depends on jQuery, I see compass becoming an essential tool for extending the capabilities of the platform.
@Ozh - It definitely is a dependency, but it won't impact anyone downloading the package. Just like core is now in grunt, you don't need NodeJS to run WP, and, like @Adrian said, the CSS from core will already compiled.

I'm super excited for this and can't wait to see if they open it up for plugins to hook into core variables and slim down the number of CSS files