1: It doesn't feel like I'm playing a game as much as I'm clicking through menues. Make it more interactive, playful. Try to make the map a bigger part of the game, maybe having a flight-icon above the actual airport, and have a smaller "bubble" pop up with connecting flights, maybe try to display the target airport on the map as well.
2: Try to get coins to be a bigger part of the UI then it is. Actually, as joeld42 pointed out, the UI needs to be a tad more refined overall.
3: Achievements. Which you can share throughout the interwebs. Miles traveled are an excellent point system, make sure players can't cheat their way to lots of traveled miles, and use it to rank players.
4: If and where possible, add pictures to questions like BackPacker did.
2: Try to get coins to be a bigger part of the UI then it is. Actually, as joeld42 pointed out, the UI needs to be a tad more refined overall.
3: Achievements. Which you can share throughout the interwebs. Miles traveled are an excellent point system, make sure players can't cheat their way to lots of traveled miles, and use it to rank players.
4: If and where possible, add pictures to questions like BackPacker did.
Just my personal thoughts.