In my opinion, Windows RT/8 is the best tablet OS out there.
Here are the features that I like a lot about Windows RT/8 on tablets.
1. Snap view for true multi-tasking. Also, adjust the width of the apps in the snap view.
2. Multiple profile/user support with separate settings and apps to each user.
3. Gesture support throughout the OS.
a) Ability to switch between apps with just a flick from left. No need of double tapping the home button or tapping particular on screen buttons which will require extra effort, cumbersome and not intuitive as they claim to be. You can even use the same app in different windows in snap mode.
Also, switch to particular app with gestures.
b) Flick from top or bottom will bring the options in the current app.
c) Flick from right will bring different charms including the start screen.
Using Universal share charm you can share the content to all the supported apps.
Using the universal search you can search the app, system and web.
Change settings and operate different devices.
d) Closing the app by dragging down the app from top.
e) Swiping up on the start screen to bring all apps menu.
4. Live tiles with different sizes which show information like weather, calender appointments, latest news etc.
You can ping anything info of apps to the start screen unlike widgets.
5. Skydrive and Skype integration.
6. Bing integration and search heros.
7. Keyboard gestures like swiping the space bar to select between suggested words.
8. Semantic zoom in apps and start screen and simple things like camera access from start screen.
9. Impressive first party apps.
10. Works with all the peripherals like printers out of the gate with built in drivers.