Document preview: Metro loses
Metro has a file system, so apps aren't tied to their compatible apps. But it "knows" which apps support which file types, so when you try to open a file, Metro opens it in a compatible app for you -- there is no preview option. That usually means an Office application rather than a Metro-style app, giving you full editing capabilities but a more complex UI than touchscreen devices comfortably handle.
The Reader app in Metro opens and -- unlike the iPad's Quick Look -- annotates PDF files. It's not quite as powerful as Adobe Reader on a PC, but it's still a very nice PDF viewer.
There's nothing wrong about how Windows 8 and RT handle document previews, but they're not as flexible as the iPad.