First off, I need to say that I am a proponent of flat/minimal design when done correctly. Websites that are loaded with 3d skueomorphics remind me of something from the by-gone severely bloated flash era.
Glossy Button: gentleface.com
When creating things with a flat design aesthetic, we need to always remember to focus on our users – not on a popular design trend. So if that gradient or dropdown-shadow doesn’t help the usability factor, get rid of it… but don’t get rid of a subtle gradient or some animation just for the sake of flatness…
Flat design reminds me of the fashion industry a little. Every year a new fad comes out that everyone must wear to look cool and be popular. If you don’t wear what’s trendy – you are outta it man!