Well the entire point of icons is to be easily recognizable and distinguishable. The more detailed you make an icon, the more convoluted and arbitrary it tends to be. The more convoluted and arbitrary, the less it tends to make sense.
This is why "flat" icon design has become so popular. It basically has evolved from logo design. If you look at most logos, they're flat. There tends to be very little in terms of gradients, highlights, or "3D" design elements. This is because logos usually need to be used in a lot of different ways, from the front of a store, to letterheads, to websites, to whatever else.
This generally benefits icon design as well. And in spite of what some people may think, an icon being "flat" doesn't necessarily make it appear like it's made for a child, either. Colors chosen have a huge effect. For instance, here are some flat icons that would be difficult to call "kiddie".
https://d13yacurqjgara.cloudfront.net/users/136389/screenshots/1113492/flat_icons_brownie_theme.png
Hell, a couple of the more popular iconpackages are also pretty flat...
https://www.wincustomize.com/explore/iconpackager/2083/
https://www.wincustomize.com/explore/iconpackager/2080/
So while some of you may not like "flat" design, I have to disagree, as well as clarifying that that's just like... your opinion man... 
I will say that the Windows 10 icons are pretty ugly, imo. I think the best term for the set would be "uninspired"... maybe "generic". As for looking "kiddie", I'd say Windows XP still wins overall in that regard. Over-beveled "plastic" UIs are so 2001.
I'm currently working on an icon set to match a blind I was building. It's entirely flat. Am I to assume no one here would like it because flat = "kiddie"?
http://i.imgur.com/Qmq6DMA.jpg
If there's one thing I wish MS would do, it would be to remove the requirement of 3rd party software from the icon equation entirely. Not to slight Stardock, but to make the task of swapping out icons less monumental. I wish there were a simple folder in System32 called "Icons". Each icon used by the OS would be in this folder, and would have a specific name. To change the icon, all you would have to do is replace it with another icon of the same name and reboot. It would make changing ALL of the icons used by the OS a snap, and I'm sure people would be uploading huge packages of system-wide icon packs all over the place.
But of course, that's just an OS customizer's wet dream...