I totally agree. For example, the CSS box model sounds so simple and elegant at first, but when you add dozens of styles and elements to your page, the interaction between different styles can quickly become horribly messy no matter how disciplined you are with your CSS code. Beyond a certain number of styles, CSS inheritance turns into a black art.

CSS simply doesn't scale well for anything beyond relatively simple pages. A huge part of designing web pages is layout and spacing and yet positioning and spacing elements remains ridiculously messy and convoluted (even with CSS3).

You could also argue that tools like SASS and LESS are in effect sticking-plasters for inadequacies in CSS.

I know hindsight is a wonderful thing, but if we were designing CSS today from scratch, I think it would be designed very differently.