I like something that throws in the kitchen sink (like Buffy), that blows the budget (like Buffy), and has plenty of call-backs to the beginning for the fans (like Buffy)...That leaves you wholly satisfied, but maybe leaves a little something there for a possible future (like Buffy comics)!
EDIT: I misunderstood too, and jumped the gun with my response! I don't like internal farewells if they seem forced. The only time I think it works for me at the end of a series is if the characters are organically separating (ie. dying, or moving apart) - Doctor Who often has this with the Doctor "dying", but any show that has a character 'moving on' can pull of a legitimate farewell without seeming to staged for the audience (*turns to camera and smiles*)