The original Macintosh had no privileges system and let apps write to random bits of memoryNeither did the PCs of the time, but the difference becomes obvious when you actually try to write an app: PC magazines were filled with BASIC and Asm listings (to be entered with DEBUG), both of which could be immediately used on an IBM PC with DOS, whereas to even start creating --- or for that matter, modifying --- software for the Macintosh was pretty much a non-starter for everyone who didn't want to actually invest plenty of $$$ in it.
Documentation on the system details is barely available (there's Inside Macintosh, but that pales in comparison to the IBM PC Technical Reference series --- the latter including full BIOS source code and schematics, even for the monitor and hard drive), and of course the PC was far more expandable. Apple wanted the whole stack locked down from the beginning.