1. Desktop was far advanced to what Windows 3.1 had; full drag and drop that worked, apps/services could extend the desktop, etc.
2. Ability to run crashy Win16 and later Win32s programs separately from each other, each in their own protected memory space.
3. All this ran in 16-32 RAM. That is MB of RAM. 128MB for OS/2, which gives you a command line Linux environment or possibly a very constrained GUI, would be huge for an OS/2 install (at that time).
2 things IBM did not fix:
1. Cursor could be locked up for a while in certain cases (like the spinning rainbow on earlier OSX) - NT never had it because mouse ran in a seperate thread.
2. Microsoft ran rings around them PR-wise.