The AOS desktop and window manager are very primitive and slow, many of the current features are missing, and I find this a pain.
Most Features you might be missing can be installed with commodities. If you are missing something like mac´s QuickLook - well make multiview a hotkey.
As for speed - on comparable hardware you won´t find anything that is faster than Workbench.
What is your favorite WM? I used WindowMaker in the late 90s and I think it is among the best of them. I have however never found a filemanager that satisfies me. Also many X11 software really isn´t designed for click to focus or focus on background windows...
it's actually quite important to be able to configure such a system to work in the way the user wants.
But neither KDE nor GNOME nor Windows nor Mac OS let you even control the Z-order of your windows properly. You can´t tell Windows to open two files of the same type with different programms. You can`t leave out icons on your desktop or snapshot a window or icon. All this is just on Workbench and I bet if someone implements "modern" features into Workbench, he is going to break some of the old stuff. Why not let these be commodities?
I don´t get your meaning of "modern" anyways. Or do you mean multitouch?