Features:
Aros:
I think the main advantages (between others) are that it is available for different hardware (X86, ARM, 68k and I think PPC) and even hosted (f.e. on Linux what is a nice option :-) ) and 3D support.
I try but I am no expert for AOS and MOS...
MorphOS:
MUI4
Ambient (or similar)
Poseidon (already in MorphOS and Aros)
and a lot more I do not know :-)
AOS:
I do not know much about AOS to say any features because I do not use it. I know that there are many components done different like Reaction instead of MUI, another USB-Stack...
3.1.:
Most things you can do in 3.1. you already can do in any of the NG-platforms. I think main advantage is the huge software-base, so tons of software :-)
What I think what should be there:
common drivers (USB, PCI)
cross-platform support (including hosted versions)
cross-plattform development tools
common software
API should be equal (as far as possible), GUI can be different (like KDE or Gnome)