Flat icons may not take as much effort to make (which is debatable... in some cases, they take even more thought to create since you have to think about every little line and curve) but they do get their point across a lot faster and more effective than a fancy, shaded, intricate icon with tiny little details. In my opinion, if you can't easily draw an icon on a piece of paper in a few minutes, it's not a very good icon- it's more of a picture or a drawing.
And I'm going to stop here, because people are going to downvote the hell out of me. Design can be something very divisive.
This (blue + simple, light shape) trend was taken to such extreme that nowadays I feel discomfort looking at the screen with FB, Twitter, Google, LinkedIn, IFTTT, Dropbox, Skype etc.
> Your mind shouldn't have to interpret colors, light sources, shading, depth
You have the cart before the horse here. Your mind already does that. There are lots of perceptual tricks that designers can use to enhance their work, all due to the way the visual system works. Certainly an icon can be too busy, but sometimes adding those items makes things easier to digest. For example, consider a 'buy' button that's a flat, single colour, versus one that has a gradient that makes it 'pop out' from the screen a little.
In my opinion, flat design was "invented" as a cost saving measure. In fact the design of all other things around us follows a similar pattern - complexity and "vestigial" features are being reduced.
Maybe I am cynical, but it's easier to hire couple students filling outlines all day than to have proper artists that can do coloring, shading..
This happened to architecture and interior decoration before. We have a nice railing in my house (Prague, middle class appartment block from 1890s) that would be too expensive to replace today, you would be lucky to find a blacksmith who can do that.
Apparently, it was a normal thing 120 years ago. How come that with all the technology we have today, we can afford intricately crafted designs less and less? My only explanation is economic - people are cheap and so-called "simple" design is good enough. I think we are losing something. Instead having people skilled in craft we have them either unemployed or having some bullshit office jobs (I am no exception btw).