Probably a lot of things. Often software is simplified, at the time because of limited hardware and probably other software. Nowadays it's often a deliberate product decision but it seemed for OS/2 no such limits existed. E.g. you could right-click on a program, get the properties, run multiple applications. It even had a Windows emulation so stable that it was never matched by WINE. Of course there was only 16 bit Windows support but still...

Of course it had limitations of its own, I don't think you could any DOS/4GW games. Linux Installation seems simple compared to installing OS/2. I had to go through some sort of pre-installation guide which was printed on a separate paper and not part of the official manual. Also dual boot was meant literally: you booted into OS/2 and then you could "exit" into Windows. Back in DOS/Windows there was a command to do this the other way around. One time I didn't do this for half a year and was really anxious if my setup would make it...