"Flexible date search" is SO key. And being able to do an extended flexible date search, over multiple months, is great (like travelocity and zuji). I don't like how some search sites will only let you do a flexible search over one month, or worse over just a few days. It's really helpful when planning when to take a trip. I won't use sites that don't have flexible date searches.

In terms of interface, ITA's "graphical" view is wonderful... by far the best way of displaying fares I've seen. It's much better than just having the times as text. You could maybe have a different coloured bar for each fare type (in addition to writing out the fare type, of course)