So if the OEM didn't install anything aside from a completely standard (albeit old) Windows 8 install, then this is Microsoft's intended behavior of Windows? What else would cause any issues with the OS out-of-the-box aside from OS problems if the OEM didn't add anything?

Drivers.

So everyone else talking about how they nuke the OEM install and do a clean install of retail Win8 aren't using the drivers from the OEM, but the standard MS drivers? I guess then my question is "why aren't the world's shittiest drivers written by the OEM that affect system stability and performance considered OEM bloatware when the MS drivers work just fine?".

It's Microsoft's fault that this is all it means. "Signature" could (and IMO should) mean that they've worked with the OEM to ensure everything works well and stuff is up to date. If Microsoft decided that Signature just means "slightly less crappy than usual" then that's their own fault.