Lou Rosenfeld has done some thinking about site search and what it should deliver. Useful thoughts. Capsule summary:
Locating search: Where is it?
Scoping search: What will be searched?
Query entry: How can I search it?
Retrieval results: What did I find?
Query refinement: How can I search some more?
Interaction with other IA components: Can I switch to browsing when search isn't doing the trick?
Finishing search: What can I do now that I've done searching?