One major, technical differences from PCs, is the uniformity of the hardware. This is becoming less true, but consoles traditionally have fixed hardware. No "works on my machine" problems on consoles. This also guarantees stable performance characteristics, that developers can optimise for. (This is less true now that consoles are resembling PCs more and more internally).
This is even more visible on older consoles: take an N64 (PS2), plug in a cartridge (insert a CD), and voilà you have your game, completely separated from any other program. Perhaps one of those programs could be GNU/Linux, but the default would still to be running on the bare metal, without interference from other programs. Quite unlike the PC there.
Incidentally, I could see a new game console solve the Thirty Million Lines Problem. https://caseymuratori.com/blog_0031 Fixed, powerful hardware with a well defined interface could possibly trigger the OS competition that is so sorely lacking rights now: Windows, Linux, MacOS, IOS, Android, and if you pick a particular niche (Server, Mobile, Desktop), you'd rarely see more than 2 significant contenders.
For example, is my microwave a general purpose computing device just because I can upgrade the firmware, even if the firmware has to be signed by the manufacturer?
For instance: the iPhone. It would definitely be general purpose if you didn't have to go through the App Store™.
Your microwave oven is different: minimum input, minimum display, one main purpose (heat food). Properly constructed ones can easily be bug-free on the first try, no need for patches. The firmware may even be fused into a strictly read-only chip. Clearly single purpose.
Personally, I'd tentatively set the limit at programmability: if there's any way to reprogram a machine, the user should be able to do it without authorization from the vendor. (We could make exceptions, for instance break control software in cars: such software should probably be tested to death and vetted by regulation. Preventing users from rolling their own may be justified to avoid untimely deaths on the road. Though "preventing" here could mean "legally disallow" rather than "use DRM". Not sure which is best.)
They run authorized programs, not arbitrary programs.
What is the downside?
