The UI is too complicated and confusing. I can't get my head around it. Can you simplify it?
Zooming in and out of the map with the mouse wheel doesn't work.
Too many error messages. Instead of saying "you didn't pick any destination", ask me to pick one from a list or predictive textbox.
Don't show me destinations to which there are no routes. Spending five minutes clicking around to get an error message in the end is no fun.
Don't separate the to and from locations so much visually. Tabs don't feel right for that kind of thing. Anyway, it's just two fields, just show them together.
You didn't completely carry through the concept of going from place to place, rather than airport to airport. When I search for Paris as my destination, you show me a dot on Paris the city, and couple of dots for the airports. My instinct is to click on the dot for the city, not the airports. Let me select the dot for the city.
You could make this really intuitive by getting rid of the text boxes entirely. Let me just click on the map to do everything. Click to select both start and end locations.