The text fields are huge, meaty, clearly-labeled things. Easy to find and click on. Instead of being relegated to a forgotten sidebar, the search activity itself is the focal point of the page. There are no distracting promotions or other crap you don’t care about. “You’re here to search for your flight, so let’s make it happen!” cries Hipmunk, grabbing you by the cheeks and shoving you into search land. Want to leave tomorrow? Type “tomorrow” into the date field.
For reference, let’s compare to another site.
Look at that shit!