A good partner:
Communicates, with me, the players, and the coaches
Doesn't steal my calls, but is there for when I kick them
Enjoys the game
Is fit enough to keep up with the play
Knows the rules
Acts professionally and is punctual
Is willing to listen to my opinion
Remembers that the game is there for the players - not us
Doesn't showboat
I think that pretty much covers it.