Windows RT is a pure tablet OS. there will be no desktop.
i am not sure wether there will be any file manager for Windows RT. i was sceptic about iOS having none, but seriously: i can not think of a scenario where i would want to use a file manager on the iPad. a tablet is a different device than a desktop.
why would MS not allow different browsers? most of you guys on WC hate Apple with a passion, but even this "evil" company allows plenty of alternative browsers on iOS (not different rendering engines though, with the exception of Opera which prerenders sites on it's own servers). Chrome runs great on the iPad and especially the synchronizing via the cloud is fantastic.
but as i have yet to meet one single person with a Windows phone or a Zune i am pretty sure how much of a success Windows tablets will be. 