Well, there's nothing inherently wrong with the syntax or structure of CSS (and things like LESS make it even nicer).
I'm talking about things like the way "floats" transformed from a way of wrapping text around pictures (good) to a general-purpose layout mechanism (disaster).
About how there's no way to put a footer at the bottom of your page that stays at the bottom if your content is short, but gets pushed down if your content is long (the normal desired use case).
About how the only way to get two columns of equal height, but that expand to fit the text, is to use a <table>, but that this is supposedly disallowed because it's "unsemantic".
About the complete disaster that float-clearing is, yet how it's central to almost all modern CSS layouts, and fixed only by bizarre byzantine hacks like "clearfix", overflow:hidden (which breaks other things), or the classic "<div style="clear:both;"></div>" which needs to be peppered throughout HTML, negating the whole supposed distinction between HTML and CSS.
And finally, the fact that the width of an element doesn't include its border and padding, making for nesting div's that include border and padding much more difficult than it should be. (Although this is, of course, fixed in newer versions of CSS.)
In the end, it's not really "current language features", but mainly the disastrous decision to adopt floats as a basic layout concept, instead of grids/columns/etc. (Which is, of course, why so many people wind up using <table> in the end, which has its own problems...)