I completely agree that Bran as the Big Bad needed more work (if that's what they were going for, which I doubt) but everything needed more work to make it work this season. I think that if Bran actually were being possessed by the Night King we would have got a flash of blue eyes, or something like that, but I think the ending is meant to be taken as it looks, the Starks triumphant, but also separated forever. (I think that's the thing that's meant to make it bittersweet?)

It's good thing we all have a lot of experience with shows that totally fall apart halfway through, right?