An excellent 2 part article by Jared Spool of uie.com on what makes effective search results pages.
Having watched hundreds of people search on numerous sites he draws out attributes that make some search engine results pages more effective than others.
The bottom line is that you can't just trust the technology driving your search engine to deliver an effective service. Understanding your users and tweaking your results pages accordingly is the route to ensuring the most visited part of your site does the job you want it to.
Producing Great Search Results: Harder than It Looks, Part 1
Producing Great Search Results: Harder than It Looks, Part 2