On the plus side, OS4.1 has hardware compositing using the 3D hardware, that neither 2000 nor XP had (Vista was the first MS OS to do this). On the minus side, OS4.1 lacks resource tracking and memory protection, which make Win2000 and XP a lot more stable than OS4. I know this is going to be controversial, so I'll clarify: if you run a program with a bad pointer bug (common in programs written in asm, C and C++), in Windows the MMU will catch it and Windows will pop open one of those infamous "an application has performed an illegal operation and will be shut down" windows. In OS4, a similar program will corrupt the state of some other random task (that could be anything, including a device driver), and if it crashes it will leave windows open and/or lock up Workbench requiring a reboot.
I will say this: OS4.1 compares *very* favorably against Windows 95/98, but to do well against the NT based OSs we still need those two features.