Honestly, if I was presented something like that site I would ask if they cheaply treat me as a kid. Colored flatness went too far, imo. Where is good old design, guys?
Good old design work has embraced flatness for ages -- literally, you can find flat design throughout the 20th century. It's also considered the more elegant by several design schools.
There's nothing about gradients, bezels, and such that makes them more "mature". If anything, it's the opposite (as flat can do away with often superfluous decorative stuff).
