Windows RT could only run metro apps. This can run x86 apps on ARM
Windows RT was only allowed to run Metro apps. We'll see how much of x86 we're allowed to run when we get our hands on it.
"Allowed" is a funny way of phrasing this. The technology Microsoft has built to run x86 Windows apps on ARM didn't exist when Windows RT was released.
I think that "allowed" is correct. A developer couldn't just recompile their desktop-style app into an ARM binary for RT, for example. They'd refuse to run, and not because of the CPU architecture. The only way to get third-party apps was through the Windows store, which only supports Metro apps. Metro apps had a limited API available to them.
I think the store now supports win32 binaries too.
Might now. Didn't in Windows RT.
Can't help but think this was just a marketing error. Had they called the OS something like SurfaceOS instead of WinRT, perhaps the expectation would have been something more accepted, akin to iOS and MacOS.
Sadly they keep making that mistake by pushing for "Windows" for their mobile OS and as you mentioned, ARM OS as well. They really should not try to push "Windows" where it's not really wanted, but instead re-brand for other platforms, and allow it to all come together.
Before Microsoft closed the door on it, it was possible to run non-Metro apps on the Surface RT desktop.
It's possible to run unmodified x86 code on this. That was absolutely not possible on RT.