A 'Modern OS' is quite simply defined as one that runs on 'Modern Hardware'. I don't think it has anything to do particularly with what features the OS itself has. And yeah, while the Operating System may come with Notepad, or some basic text editing, I would still call that an application, and not necessarily part of the operating system.
The only thing (in my mind) that makes the AmigaOS (and all of their brethren, female companions? what would be the female word for brethren anyhow? Since the Amiga is a feminine word.) not a 'Modern' Operating System, would be the fact that it doesn't work with modern hardware. Yes, that's even including the AmigaOne X1000. While the hardware itself is more or less modern, AmigaOS4 doesn't FULLY support it yet. Making it not a FULL (modern) operating system.
I absolutely love the AmigaOS, and while I agree it does need something as important as memory protection, multiuser and security, it's not necessarily required for a 'modern' operating system. But you do need support for whatever hardware it's going to run on. If you have a Dual Core PPC and AmigaOS4 only supports one core, that's not full support. The key word here is SYSTEM. You can have the most epic Operating System in the world, but if you don't have any hardware to operate it on, then well.... But then again the same goes the other way. You could have hardware that would make a basement dwelling troll go on a 'fapping' spree, but if you don't have an operating system that doesn't suck....
This is the conundrum of 'Modern Computer Systems.' In my opinion some of the ways AmigaOS did things were just beyond anything we have on 'Modern Operating Systems'. Screen drags and different resolution screens on the fly! I mean that is still one of the coolest things I've ever used. Granted with LCD/LED screens looking completely horrible unless you're using the native resolution (or perfectly divisible of that native resolution) then multiple resolution applications would be a bad thing, but I think they still have their use. Another one would be custom icon sizes and clicked on and non-clicked on icons (damnit, I want my folders to look opened when I single click them!)
Sometimes I dream of being able to run something as light weight and fast as AmigaOS 3.x on my 8 core AMD Bulldozer with 8GB of RAM and being able to utilize all my hardware. It would absolutely SCREAM!
Unfortunately by my own definition though, even AROS isn't a completely modern operating system, because it only runs on specific hardware still. I haven't managed to even get it to boot on my laptop or my desktop.

slaapliedje
P.S. Sorry for the rant...