Just some initial feedback:
1) I want to specify an airport / dates before searching. I live in the DC area where there are 3 different airports prices could be from, but only like to fly out of one.
2) When I try and look at a destination, and find flights, I cannot seem to find where to add the dates, which it complains about not having.
3) Scroll wheel doesn't seem to affect zoom on the map at the location. That's taking away an expected feature of the map, and not something I like.
4) Number of people up front along with total budget. Sharing hotel rooms makes thing much cheaper where hotels are expensive. You could have a more expensive flight and go somewhere more interesting for the same amount.
5) Once I am on the results page, I might want to filter out specific countries and not just whole regions. Or I might even take a whole region, but I can't seem to add a filter after the fact.
6) Why does the 'Home' button open a new tab? If I am clicking home I want it in this tab. In fact I always want it in the current tab. I will 'Open in new Tab' if I want it in a new tab.
7) You have a way to get reminded about the site for later travel, but what if I want to book now for a Spring Break trip? How do a search for a trip 2-3 months from now? I am not planning a trip like this site is useful for last minute.
8) Maybe its just me but 'Return' flight is not what I am used to calling it. Normally that is referred to as a 'Round Trip' flight.