Excellent answer. To which I'll add:
IFF standards. In today's world it would be an unthinkably open approach taken by open source only. Even more surprising it came from a joint venture between Commodore Amiga and EA! Every single graphics program understood, and via datatypes understood in a standard way, IFF graphics. Saving, processing or reading. They were so prevalent and expected that you would be hurting your chances to release something with a propriety only format. Same went for sound and no end of other things. Had the Amiga thrived there'd no doubt have been an IFF in place of many of the multimedia formats. With a standard OS level library call to decode them, etc.
The conciseness of approach, necessary in a system providing proper multi-tasking in 256K, meant all the services other platforms placed at least partially in the .exe were usually in the OS. There were system libraries you could rely on without the absurd version dependent dll hell of windows. I'm sure had the Amiga persisted there'd be some version annoyance, but I can't imagine it reaching the stupidity of now.
Windows had far more than glue code in the exe. If you needed to use a file requester, accept messages, have a window that could be resized etc there was tons of unique OS related code in the exe for all that etc etc. Update the OS and unless you update the source and rebuild the exe it will clearly and obviously be of the previous Windows release. Or as was so often the case buy the latest office and the look is clearly of the next, unreleased Windows. Amiga had it such that all that rubbish was nearly all external. Set up your structures, call the API, get woken up when there's something you need to care about. If you updated OS, all your window chrome, file requesters etc, would be of the new OS. No ridiculous dependencies on v 3.2.152 of MSVC.dll, and 3.3.x not being acceptable, meaning you end up with 12 different installed versions etc.
Only apps doing something clever - like CygnusEd with its hand written assembler scrolling that remains, 30 years later, my benchmark for "fast and smooth enough" editor scrolling. Essentially nothing has yet matched it, though Sublime is probably closest just without smooth scrolling. It was really difficult to come to accept - in some sense I still haven't - that I had to do so much of this OS housekeeping for myself each and every time, in every application for other platforms. I often used to wonder what Windows was, in fact, adding as it always seemed like I was doing everything myself. I gave up complete on Windows programming pretty quickly as a result. :)
AmigaDOS may have been a bit of a last minute, ugly addon, but in use it felt like a lightweight single user *nix. Proper filenames, priorities, comments, proper scripting and ARexx if you needed additional integration. Sure, it was far happier on a HDD, but what aside from DOS - more a program loader than OS - wasn't? :)