P-J... are you sure that you're talking about Operating System, and not the underlying hardware?
Because the HARDWARE of even your shitty Dell, although not as intelligently working together as the Amiga's, is way faster, and THAT is the reason even Win95 can produce results we can't see on the amiga without some very very clever coding tricks. (and fooling the eyes into believing we're seeing the same things).
The Operating System itself... there's only two things on the top of my head that the amiga lacks, and that's memory protection and virtual memory.
AmigaOS3.9 does everything I need it todo, however, the hardware is too slow for me to run WoW on my A1200. THAT isn't the OS' fault. That has to do with outdated hardware.
Please differensiate between the OS and the Hardware.... evne he worlds best OS won't run without anykind of hardware. AmigaOS3.9 is, IMO, just as good as OSX and WinXP. it just happens to be intelligeently coded to make the most out limited resources, while OSX and XP/Vista has a gazillion more resources and the coders doesn't need to worry, so they just throw out ideas and say "get a powerful enough CPU and you may be able to run it"