Loss of affordance, density and contrast. Why do designers hate these 3 things? It's pretty, yes.
(Interfaces are to be printed out and to be discussed while looking at them mounted onto presentation boards. This is their primary purpose nowadays. – API's have still a way to go in order to catch up. E.g., we're still using calls like "Performance.measure()", where it really should be low contrast "Pink.mint()", which is also much prettier in a printed list of calls.)