I know I'm not making any big revelations here, but the funny thing is there's no "plain text" at all. It's just that our tools all know how to decode ascii. ascii bytes are just as non-human-readable as any other until they're decoded as ascii and displayed on screen. Another encoding could be just as transparent if all our packet sniffers, editors, etc, spoke it.
This here is really the only revelation that's ever made me think twice about text-encoded protocols. Someone else here mentions TCP, IP, etc, and it's true that it's pretty readable in Wireshark, when you spend a little time learning where to look for the information. BUT... You still essentially need Wireshark to read it.
