KnownFolders doesn't work on WP8 for 3rd party apps.
In terms of access on WP8 here's what's available:
- Contacts: Read access available since WP7.5, write-access available via WP8 ContactStore.
- Appointments: Read-access available since WP7.5. In WP8 you can add individual appointments after user confirmation via the SaveAppointmentTask.
- Photos: Read access to all folders available since WP7. Write-access to Camera Roll and Saved Pictures available since WP7.5.
- Audio: Developers can iterate over the music library and even play it via the native player since WP7. Starting WP8 developers can add songs to the music library using MediaLibraryExtensions.SaveSong().
- Video: Read-write isn't available. There are security and storage issue with transporting files that big.
- uSD Card: Starting WP8 apps can register for specific file extensions and read those from the micro-SD card.
- Documents: No read-write access. But starting WP8 developers can open up docx/xlsx/etc files using Launcher.LaunchFileAsync and it will open up a read-only copy in Office. Users can then choose to save that copy in the Office Hub.
I've made this post a wiki so if there are any additional areas, feel free to edit and add those bullet points.