PS: I'm the author of this article. Had a great time thinking about all the fun systems I've played with over the years and researching their history.
* Built in BASIC on the Commodore PET
* Folders in MS DOS (CP/M didn't have them)
* The trashcan in Macintosh System 6
* The start bar in OS/2
* I can't think of a single unique thing about Atari TOS
* The stability of Windows 2000 (compared to 9x and NT)