Lync Tastatur snarveier
You can use these shortcuts when any window is open:

Windows logo key+A Accepts an incoming invitation
Windows logo key+X Decline an invitation and change status to Do Not Disturb (my favourite)
Ctrl+Alt+Spacebar Take back control when sharing your screen
Ctrl+Shift+S Stop sharing your screen


Lync Main Window
You can use these when the Lync main window is in the foreground:

Ctrl+1 Go to the Contact list tab
Ctrl+2 Go to the Activity Feeds tab
Ctrl+3 Go to the Conversation list tab
Ctrl+4 Go to the Phone tab
Alt+A View all conversations when in the Conversation list tab
Alt+D View missed calls when in Conversation list tab
Ctrl+1 or Ctrl+Shift+1 As a delegate, transfer a call to another person’s work number

Conversation Window
You can use the following keyboard shortcuts while in the Conversation window:

Alt+l Invite a contact to an existing conversation
Alt+F4 Close the conversation window
Alt+D Decline a file that has been sent
Alt+P Open a file that has been received
Ctrl+R Show or hide the participate list