CLI commands in English - easy to learn, something unix (and like) systems seem to completely ignore.
Responsiveness - very few others can even get close, the only one I can think of is BeOS. This is a design point of the Amiga which again others have not bothered with.
Simple system layout - again something other systems seem to completely ignore.
No swap - I consider the ability to run *effectively* without swap a major performance and resource efficiency advantage - it means you can build a system without a HD, try that on any other system... Yes, you can build a minimalist Linux system but you'd lose a lot of functionality in the process.
Note: The lack of VM is a distinct disadvantage but VM is not the same as swap.
Title bar at top of screen - Ever since going over to the Mac I've become convinced this is a good idea, however the mac implementation is better as it does not require you to press the right mouse button.
A standard scripting language - the fact of a standard meant everyone uses it, the lack of a standard scripting language on other systems means they are not used even if their capabilities are better.
Experimental GUIs, this is difficult on other systems but Amigas embraced it back in the day, it didn't always work but some non-standard apps had excellent interfaces.
DOpus 4 (not the later versions). IMO the highest point of file management applications.
History - many ex-Amiga users have fond memories of the Amiga, this is not true for pretty much any other system out there today.
