I happen to think it looks ugly and I find it difficult to use (seriously I find using Windows Server 2012 incredibly difficult to navigate visually). I think these flat UI sites look unsophisticated and badly designed. They remind me of 1990-era user interfaces, and not in a good way.
But that's just my opinion.
What's great about the Metro style is it makes responsive layouts dead-simple and it cuts the design process down considerably. Sure it looks boring, but that's okay for now because I spent years hearing about wet floors and jelly buttons and skeuomorphism and...
It's refreshing to be able to pick a few colors, pick a few fonts, put it on a grid and be done. To me it almost feels like laziness, but it's a nice change of pace from the days when a single button took hours to design/build.
Miracle of TV being broadcast invisible over air, then we start putting cables everywhere. Terminals and servers evolved to desktops, and now we go back to servers, or "Cloud". We had a Space Shuttle which was reusable and was plane like, now we ditch that and go back to olde skool rockets. Now this web design style, which to me throws out everything we were taught about usability, etc in the past.
No, not precise exact parallels, not with out good reason either, but to me its a broad, vague pattern. We seem to eventually loop everything backwards eventually.
Not saying this is wrong or bad, it just seems like what "we" do. Its sort of like music revivals, fashion revivals, or TV show remakes.
Look out for building block sized phones and black and white TVs in the future!!!
Oh, hang on. Phones... Used to get smaller and smaller, now smart phones are huge. There you go, another one!!!!
I notice that these are not in contradiction with adjectives that come to my mind first.. such as sterile, pretentious, mediocre, repetetive, heartless. Though now I think I'm kinda projecting properties into "flat design" it might not have, based on the properties I see becoming real hip in people.
I'm really curious what the general public's perception is (not that it really matters) and if there are really any functional implications for the way some of the interfaces have been designed.
What's missing? What other more appealing, and non-gratuitous/extraneous, forms may follow the function?
Gradients, drop shadows, bevels and other 3D effects function as visual cues, and can help a user figure out the hierarchy, order and function of elements in a 2D design. They're not extraneous, at least not when used properly.