One thing that Kayak is doing better than Hipmunk is multi-airport things. I.e. We have here in Silicon Valley SFO and SJC. I usually prefer SJC, but can fly from SFO too. Some flights are cheaper or have less connections if you depart from SJC but arrive to SFO, or vice versa. I couldn't find how to tell Hipmunk that it is OK (or not OK) to use different airports for round-trip. Kayak does it very nicely. It also seems pretty hard to say "I like these dates but wouldn't mind to go day earlier-later if that gets me couple of hundreds of dollars back".
When I search for flights home (Boston) from Santa Clara, I usually do a search for "SFO,SJC" to "BOS". This shows me all flights from SFO and SJC to BOS allowing me to pick and choose which flights work for me (time wise) as well as what makes the most sense price wise.
As for the multi-dates, when you select a date, you can click the "+/-" buttons to add time to your query. You can also manually add "+-n" to your date to give a range.
So, say I was open to flying out July 12, 13 (original), or 14 - I could update my date to look like: "Jul 13 -+1"
