Fast boot (don't care what you say AlexH my 1200 always booted in 15 sec with a crappy old IDE HD, and my XP with two disk raid 5 still boots in times measured in whole minutes)
The transparency, couple of e.g's: Startup-sequence for stuff that's starting up on boot, Libs go in libs: devs in devs: etc (compare linux - is it /usr/local/bin or /usr/local/sbin or /var/this that and the other? argh I dunno!). Snoopdos would weed out what was missing in seconds (try that on Windows - bazillions of OS calls per second) Libraries that were ACTUALLY shared (seen how many .dlls on your win system?)
Robustness IS on my list - basically if you had an app that twatted into the memory of others it did not get used for long - forced better programming and no enourmous waste of CPU checking the memory accesses were allowed.
That said - Crashing also very nice. Crash, quick guru, reboot, back to where you were. Better once they remove the necessity to press the LMB
User interaction - having the OS drive a programs' GUI and not the program itself was genius - made the system feel much more responsive even when it wasn't - the screen updated even if the app would not. And none of the system thinking you clicked on something that wasn't even there when you clicked (even bloody tomtom sat navs do this!). None of the window repainting problems that win STILL suffers with either.
Multitasking - I still maintain I have seen NOTHING that can genuinely multitask as well as the Amiga OS, although I am REALLY not sure why that is. As a small example, look especially at the apps that do graphics/video. You will see stuff running at 50fps and NOT juddering, jerking, pausing for thought. Even video display systems run by very fast PCs as so often the case these days will run at 33 1/2 fps or something slightly out, and every now and again they'll pause briefly for a little think when something complicated happens - like maybe accessing a file on a share or something
Files being identified by what's in them rather than the three letters on the end. I thought this was SUCH a good idea and really can't understand why it's not more prevalent.
Fast off. Make sure your files are saved, wait a sec, and power the thing off. None of this "shut down" bollocks. "Shut down" is NOT a task that I ever WANT to do!!
Yeah I agree with some on here, the addons to make AmigaOS come into the '90s (never mind the '00s) have made it flakey and unreliable. TBH that's probably why windows is so bad - it's still fundamentally the original thing with millions and millions of addons bolted onto it. The original underlying OS (up to 3.1 I'd say) was still very beautifully done. It astounds me that a 50mhz 020 Amiga with 16 meg of RAM can feel more responsive than a 2ghz dual core beast running Vista.
Wow that turned into something of a rant. sozzer about that