I think that you will find many fewer deisgners hanging around these thing than photographers (self interest I supose).
But as a designer, I really like Fotolia's category systems. I have heard the photographers complain a lot about it, but the conceptual categories have helped me find the right image several times.

Most of the times you will find the image you need just on a keyword search. But sometimes the "idea" has no key word. What then? That is really the only time that the categories are any good at all. I have seen a lot of stock sites just kind of copy each other and add a few or something like that.

Anyhow, just wanted to answer your question, "What is designer looking for in a stock photo place"

PS: I also look a lot for illustrations, not because I can't do it myself, but because at $1-$5, why would I

::butterfly