One should always be cautious about simple design rules. As design goes, in one setting the flat, grayscale approach can be dull, in another one, it can be cheerful and captivating.

I have a recent example for the new mantra of removing color and flattening things. Take Sourcetree, which is one of my favorite applications. A few weeks ago they’ve switched to a new icon set for their main icon bar:

before http://www.sourcetreeapp.com/img/screenshots/st_feature_hist...

now http://www.sourcetreeapp.com/images/sourcetree_hero_mac_hist...

I don’t think there is any usability or aesthetic benefit to it, quite the opposite. The original version had something slightly funny and frivolous about it, which made it pleasant to use. And from the usability perspective, one could subconsciously click the yellow icon without examining the picture itself.

There are also many counter examples where grayscale and flat design make things a joy to use. I used to do DTP for a black and white newspaper and we never had the feeling it was dull, quite the opposite, we considered it quite elegant and calm as compared to the flashy tabloids.