After reading other comments in this thread, perhaps the best aspects of the Amiga that are really missing from computing today is a tightly coded, efficient, responsive OS.
Windows is just too bloated, insecure, and based on an outdated codebase. I wouldn't be surprised if Windows 8.1U1 still contains code that was written back in the Windows 3.1 days, or even earlier.
Linux may be quick and efficient, but it's far too complicated compared to AmigaOS. It's just not newbie-friendly and it probably never will be. I know that distributions like Ubuntu are supposed to make it easier to use, but you've still got the same underlying architecture. In some ways, Linux has its own version of DLL Hell, which is even more complicated to deal with than Windows. And yes, I have used Linux in the past and it's frustrating to deal with when some library fails to update and basically dorks the OS up so bad that you can't roll back and end up having to rebuild the machine.
MacOS is basically Linux (it's really based on BSD, but could be considered a cousin of Linux) with a custom X Server and GUI slapped onto it. I haven't really used MacOS, so I can't speak to whether it improves on either Windows or Linux, but MacOS's biggest disadvantage is probably the price of the machines it runs on. It's not as outrageous as A-eon's X1000, but it's more than just a white-box or home-built PC. I would say though, that Apple's iron-clad control over the hardware and OS probably is a plus in its favor.
So, I guess for me, the hardware isn't so much the determining factor, but having a fast, efficient, user-focused OS that doesn't have a crap-ton of background services running that the user doesn't know what the heck they're for, might be the place to start.
I remember showing a former co-worker, a guy in his mid-20's that was probably still in diapers around the time Commodore went kaput, an Amiga 500 I'd bought off eBay and he was astonished at how fast the machine booted and the OS responded, especially when I told him the machine was only 7MHz.