the @mixin delineation is quite deliberate. See Nathan Weizenbaum's comment at the bottom of: http://nex-3.com/posts/83-sass-and-less#comments
Also, one of the most powerful features of Sass today is @extend, which is something that Less definitely does not have: http://sass-lang.com/docs/yardoc/file.SASS_REFERENCE.html#ex...