Socks!!
By the end of my pregnancy my calves, ankles and feet had swollen up.so much that I couldn't even wear DH's socks, everything cut in far too much. This also gave me a problem with trousers - anything skinny stood no chance of fitting over my legs. And I agree, it was difficult to find smart skirts for work. And don't get me started on shoes...
Maternity tights are pretty poor, either much too big and falling down at the top, or much too short with the crotch coming down between your legs.
I agree that materials tend to be cheap and don't wear well - that's a tough one though as I didn't want to spend a fortune on clothes that I wasn't going to use for very long. I had a couple of lovely Isabella Oliver things that I got in the dale but I couldn't justify buying them full price.
Tops being too low cut (esp. Nursing tops) so that they show your unattractive maternity / nursing bras.
Not having stock in store to try on. With Mothercare and H&M as notable exceptions, I spent a lot of time waddling to the post office to send stuff back.
Other than that, maternity wear is great! 
Oh, one thing that was good - I had a pair of jersey trousers / yoga pants that had adjustable elastic in the top so they could grow and shrink with me before and after birth.