-WiFi WPA/WPA2 support
-complete USB 1.1 & 2.0 support
-complete shell support for explorer_new.exe
-DirectX 9 support and OpenGL 3.0
-reasonable driver support (printer, discrete graphics, on-board network, on-board sound)
I think the list of Black Fox is excellent, and the two things I'd like to add would be:
- that "printing subsystem" you mention as necessary (he mentions it from a driver point of view, I mention it from the point of view of "black letters on white paper, I don't care how");
- an SSH server.
(I've been looking forward to this last one for years and years...)
I cannot stress the importance of drivers and real hardware too much - VirtualBox is mastu...
PurpleGurl is right that ultimately, you cannot ignore the major hardware advances of the recent years, and namely:
- 64 bit. 4GB of RAM is OK this year, but already smallish (8 are standard), 4GB of RAM will be ridiculous 5 years from now.
- Graphics card support, stuff like DirectX and OpenGL. New, particularly "flashy", apps, make use of it - it is not "just for games".
- Same applies for multicore.
If you ignore these, you will end up like FreeDOS at the latest three to five years from now.