It's zen like simplicity compared to over bloated monsters like Windoze Xp and Vista. The fact that everything works like it's supposed to and it doesn't come with a lot of useless junk that takes forever to load. It is extremely light on system resources and leaves most of your memory and hardisk and CPU cycles to you to do what you want with them.
Amiga DOS is like a stripped down Unix derivitive that is elegant and functional and has a lot of power. The Workbench GUI is a little eccentric but once you get use to it, it really rocks and has a real creative edge and it never gets in the way of things like Windoze does.
The only OS that I can compare it to is OS2 which came from the opposite extreme--IBM, Big Blue--of computer culture but had a lot in common with it including the Rexx language and the stability and reliability factor--reliable enough for the banking industry. The Amiga OS was reliable enough for the Broadcast Television industry.