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?

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.