I think CSS itself isn't so complicated, but the whole different support in different browsers thing certainly does make a mess of it!
One of the most helpful things I ever did with respect to CSS was read the apress book on html/css patterns (just ignore the parts that use javascript).
http://books.google.com/books?id=xq9IOomk1tUC
(I never realized that standard CSS actually operates with 6 box models!)
This looks like the best example I have so far to pass along to people asking me about css, which happens quite often.
That aside CSS is messy and complicated, lots of programmers dont understand the basics, and even experienced developers can easily miss a few very useful tips.