What makes these interfaces nice? [on hold]

I feel that material websites (such as stackexchange) have a nice design. I'm not sure how to explain it, it's like FLAT. A nice reference would be iOS 6, and then iOS 7's design. Take a look:

I feel like that design is light and flat, not sure how to explain it. This happened with Android too. I feel like it's ONE color, no gradients, flat icons, etc. But why is this nice? To be precise, what MAKES this nice? Another example would be this website:

It isn't really nice due to something. Not sure what it is. Gradients? Border radius?

Edit: I'd like to explain why this isn't opinion based. There are facts that make these designs nice, one of them is having a flat design. What I'd like an explanation for is what makes this flat design... Well.. Flat