The biggest feature changes I notice are in the me tile and messaging tile. I barely use the facebook app anymore.
The me tile will show if you have any notifications. They also added the ability to check-in, post and change chat status there.
In messaging, there is a new pivot that shows who is available on facebook chat and windows live messenger... and of course you can change your own chat status.
On any contact, they've also added a "history" pivot that shows all interaction you've had with them, whether its email, text, IM, or phone.
There's not much in the way of "hidden" features anymore. They look to have consciously avoided the old "hold down a finger" to access context menus; in fact moving many of those features to the bottom drawer. If I'm not mistaken, more things are pinable, including "new note" from one note, and playlists.
You can pin entire albums to the start menu, including facebook albums, but you still can't pin individual pics. Also, the camera roll album has a "delete contents" option available when you long-press it. The date pivot in the picture hub has a jumplist on the month.
You can now share a webpage to facebook/twitter from ie9, and share a video to facebook.
Local scout pivots include filters just below the pivot name. Tapping the map at the top shows the local scout results on the map in full screen.
Probably the most hidden feature was voice texting. Hold the home/windows button and when it beeps, say "text [person]". Works amazingly well. I had no idea about this feature until I was driving along and got a text, and it read it to me over bluetooth. Since discovered there is an option under "speech" in settings that allows you to configure when to read incoming texts to you (mine is "bluetooth only" - so when its hooked up to bluetooth in my car, it reads texts, otherwise it wont - but you can set it to always do this if you want to play with it and don't have bluetooth in your car).