Lack of ability to nest rules in CSS makes it harder to organize styles. Not being able to create mixins and such makes all too easy to have to copy-paste and memorize cross-browser solutions & vendor specific rules for certain browsers.
In short, there is a lot of inefficiency with the current system that preprocessors help alleviate.
They truthfully do help alleviate such things, but none of this is impossible nor too terribly difficult to handle with plain old CSS.
Oddly enough, I've seen a number of people recently complaining about nesting in SASS claiming it creates problems. You just can't make everybody happy.
That's not a problem, that's not understanding how it works.
Any nested SCSS is impossible to reuse outside of its parent scope. If someone is using nested SCSS and expecting to reuse that code elsewhere then they are doing it wrong. It's the same problem that exists if one were to create a class as a child of an ID and then expect to use that class outside of the ID's scope.
In short, there is a lot of inefficiency with the current system that preprocessors help alleviate.