@bondsbw: Problem is all of those names imply things they'd prefer not to imply:
"Surface OS" implies it's just for Surface
"Tablet OS" implies it's just for tablets (they want/ed it to eventually go on laptops etc. as well) and sort of implies x86 Windows isn't for tablets
"Modern OS", while the most accurate in terms of what they hop/ed RT would eventually become, is sort of an implied dis of x86 Windows as not being modern
Naming it with two meaningless characters was actually deliberate since it's a way to avoid pinning it down to any specific purpose.
But the real problem with RT is that its true purpose - to have a version of Windows that's less susceptible to malware and "winrot" - is inherently difficult for Microsoft to market as long as they're still selling x86 Windows. If it were Google they could do an ad campaign lampooning IE getting a million unwanted toolbars, etc., but Microsoft can't really do this. And this problem affects the naming as well.