Ah.. OS/2 :)
It was a lovely OS, despite its flaws. I used it both personally and professionally from 1993-1999 encompassing 2.0->4.5, plus forays into 1.3 (and bits of 1.1/1.2). The WPS was fantastic, the API lovely and clean, the documentation splendid, REXX very useful includings its infinite precision arithmetic and there was some unique and quirky software available for it, plus a welcoming and easy to access community.
Unfortunately it also got a lot wrong. The chief flaw was IBM's pursuit of OS/2 PPC/Taligent - if they hadn't bothered with that and had re-engineered OS/2 x86 it might still be a viable force in the OS world. I have to give Microsoft credit here - NT managed to create a decent design whilst maintaining compatibility with the horror that was 16 bit Windows. Till the end OS/2 was not a 32 bit clean system - the kernel has a lot of 16 bit code, 2.0 had a 16 bit GDI (fixed in a servicepack), windowing was 16 bit until Warp 3 and 32 bit graphics (GRADD) came in a service pack for Warp 3. DASD drivers I can't remeber - 32 bit came around Warp 3 too I think. OS/2 never fixed the Synchronous (*not* 'single') input queue problem although it was largely mitigated by Warp 3 FP17. Most users of OS/2 never got to use the 32 bit networking stack, either, and IPV6 support doesn't exist.
Still, let's talk more about what it got right - the generally splendid multimedia subsystem, the amazing X Server from Holger Veit, the sterling efforts of Stardock Systems with their productivity and games software (to this day I still play Galactic Civilisations 2 for OS/2 occasionally) and I must also give credit to Innotek who continued supporting OS/2 long beyond the point where they should have given up.. the pervasive multithreading, unparalleled V86 OS support (not just DOS - you could run other 16 bit OSes too..) but not by any means a) the progress bars that went backwards or b) the damned parrot video
I still have a heavily upgraded OS/2 Warp 4 system here, although it doesn't get much use and runs on fast Pentium 1 era hardware. Perhaps I'll migrate it over to Xen though, should be able to get it running on that..