Completely invalid advice in 2014. LESS and SASS now provide everything that CSS "can't do" in this answer.
The answer is still valid. LESS and SASS are CSS preprocessors, not frameworks. Sending spaghetti code to the user's browser can be limited but LESS and SASS do not address the flaw of CSS frameworks.
Bloat of a hundred framework rules? - None of these will be included if you use mixins and selectively mixin the stuff you need.
Unsemantic grid sizes - Personally, I think this is something where utility trumps perceived "cleanness" of semantic HTML, but if you want to you can use mixins for this as well to hide the grid stuff in your CSS.
Not responsive - SASS and LESS don't do anything specific here, but this is definitely an outdated comment - Bootstrap's responsive grid is probably better than anything you could code yourself without devoting a lot of effort to it.