The Windows RT tablets will have the "desktop" mode, but refuse to run any desktop apps not shipped with the device. You can't compile a standard Windows-application for ARM, copy it over and have it run. That's a no go.
On Android-tablets, you can whip up a terminal and execute any binary you like, no matter where it came from.
You can compile Linux user-land tools for ARM and use them as you please. In fact somebody already did that job for you so you can just deploy them on your tablet.
I fail to see how Windows RT offers anything which haven't already been here for years.
I'm not saying Windows RT is perfect or even that good, I'm just saying that this one particular thing, they got right.
Maybe stock doesn't, but install any file-manager from market, and voila you have just that.
Just like gallery-app can be a file picker, so can a file manager be. Like cm's file manager is.
Or it can be something else entirely, like a dropbox frontend. You've just got to have a little bit more imagination ;)
In Android, 3rd parties don't need to integrate, the OS does it for them via intents. That's the number one thing which makes the OS so flexible and exciting.