No system-wide file manager
No Bluetooth file transfers
No USB mass storage mode
Limited third-party apps availability
No Flash or Silverlight support in the browser
Too dependent on Zune software for file management and syncing
No video calls
No custom ringtones
Music player lacks equalizer presets
No multitasking
No copy/paste
No DivX/XviD video support (automatic transcoding provided by Zune software)
No sign of free Bing maps Navigation so far
No internet tethering support

Taken from http://www.techknots.com/mobiles/wp7-limitations/. Although MS has spoken of improving the OS through updates, it's still hard to be optimistic... Compared to the functionality of the N900 this is a huuuge step back.