The underlying things in OS/2. The mere fact you could have a hammered OS/2 server and STILL have respectable response from the GUI is an amazing feat. The task switching and swapping, scheduler and those kinds of things.
I'd like to see Linux benefit from the technology/IP/Ideaology that made OS/2 so useful in ATMs, Kiosks, Just about anything important, as it turns out, for quite a while ran as an OS2 application.
Other things like Presentation Manager would be nice to have available and usable in Linux... but I think those days are past, as in freeing Presentation manager. But WPS, still may come around some day.