When it comes down to desktops, I have pretty high demands.
I actually don’t care if a desktop is “modern” or “classic”.
I do care about functionality and flexibility.
I often regard Linux as the software equivalent of a box of LEGO® 
For me, a desktop:
- must adapt to my workflow and not the other way around.
As long as you ‘live’ inside an application, it doesn’t matter.
But if you use patchbays, converters, and multistep data manipulation from several applications, then it’s different.
Desktop designers do not have expertise in my line of work and therefore don’t have the level of expertise to be able to decide for me what the workflow should be. Dictating that is utter arrogance.
-
must be highly configurable
I sometimes assemble computers to work as appliances.
I need to be able to bend the desktop to my will because it is part of the interface. -
must have a decent ratio of functionality versus resource consumption
I’m not going to move to a desktop that eats way more CPU and RAM, adds horrible latency and gives me back less options, less functionality, less flexibility, less reliability, more unneeded complex moving parts and tries to pamper me to death.
.
If that is the definition of "modern’, then no thanks, it’s not for me.
The choice for MATE desktop has been very rewarding. It keeps things relatively simple under the hood, has the optimal balance between luxuary and flexibility and is relatively light on resources.
I use MATE for almost everything, including my mediacenter.
MATE also runs on almost anything, like my ASUS F3T laptop that is more than 20 years old
(AMD Turion X2 , 3GB RAM, nVidia GeForce Go 7300) and even plays youtube videos. 
But the desktops that I also hold in very high regard are:
-
XFCE (used it in the past)
Pretty much comparable to MATE, used to be a bit more spartan but AFAIK it has improved tremendously. -
KDE-Plasma (Ubuntu Studio)
Used to be heavyweight but not anymore, and it is also extremely flexible, attractive and fast. If I ever have to ditch MATE then I’ll probably move to KDE -
Moksha (Bodhi Linux)
Based on Enlightenment, incredibly light, beautiful, runs very very fast and snappy on my ancient <1GB netbook without ever hitting swap. It really deserves some attention and a lot of admiration.
I’m also pretty much a proponent of server side decorations for a very trivial reason:
Client side decorations make it sometimes near impossible to drag a window because the titlebar/headerbar is often so cluttered with hamburger menus, shashlik menus, chickennugget menus and buttons that there is no room left to place your pointer 
It all is, ofcourse, a matter of taste. One mans garbage is another mans treasure and I respect everyones choice in that because everyone has different demands from a desktop.
I guess my demands are somewhat unconventional.
And…oops, sorry for the long post. I think I got carried away a bit. 
b.t.w. did you know that the real meaning of the word “classic” is “the best that a culture has to offer” ?