Recently on Lou Rosenfeld's blog he mentiones the 80/20 rule, basicically focus design on the 20% of content that effects 80% of the audience.
http://louisrosenfeld.com/home/bloug_archive/000122.html
He identifies these crucial areas:
-Main page
-Search interface
-Search results
-Browsing interface
-The "found document"
James Robertson of Step Two Designs offers good commentary on"Search":
I agree absolutely with your search engine comments. I've often found it bizzare how little effort is spent getting the search interface right. Most often, IT managers install the search engine "out of the box" and leave it at that.
This links to his (James') excellent research on designing the search interface:
http://www.steptwo.com.au/papers/search/index.html
PDF
also on the site.