If I saw the "after" UI of Hipmunk without having seen the "before" UI, I would have been totally lost. Where are the buttons? What is a label and what is tappable!? It is even worse with touch, there is no hovering cursor that changes appearance. I honestly think "flat" is going to take a long time to work out the kinks. I think it is here to stay a while, for better or worse.
I honestly think people are making too big a deal about this. The kind of people who download apps are not going to have an issue exploring the UI of new apps to figure out how they work. And it's not that hard to figure out just by looking either. Verbs, arrows, and nouns that don't label something adjacent to them should all be clickable. And if they ain't, oh man, you just wasted two seconds finding that out. Not the end of the world.
Speaking as a 39 year old developer curmudgeon who routinely downloads new apps: I am not going to click around and discover your interface if it isn't obvious in the first few minutes.
As an example, the new flat google maps app is a disaster. It took me weeks to figure out how to do fairly simple things, like how to cancel turn by turn directions. My wife is totally lost, as a long time iphone 4 user trying to use the flat iphone 5 version. When she tries to navigate while I am driving, she is constantly confused while I say, "no! just swipe down hard enough that the app thinks you are going back, then click on the box, then click the x, then swipe up, and stand on your left foot!"
But we have to have a map, so we use it.
As a startup, this is what you should think about: Is your app as useful as google maps? If not, then you can't afford to make it unusable with a trendy flat interface.
Honestly, not a huge fan of newmaps, and I'm a googler. But most people I know have figured out how to use it. I did too after playing with it for a bit. People will figure things out, especially if there are no non-flat alternatives.
