MacFarlane does not appear on screen on TED or on his animated shows. The one time he has gone in front of the camera, the movie was a gigantic box office failure, with many critics specifically taking an issue with MacFarlane's screen presence and inability to carry the film. I don't know what that means in terms of how good he'd actually be in the role, but it's the kind of thing that would make executives pause before hiring him for something like this. Not to mention the fact audiences for an R-rated hit like TED are not necessarily who NBC is going for.
Neil Patrick Harris was in one of the biggest movie hits of the year, made HEDWIG the hottest ticket in town and has a contract with them. He seems the most ideal candidate (assuming Hugh Jackman is not interested). Fallon is also audience-friendly.
I'm guessing from the way they cast PETER PAN and SOUND OF MUSIC that they'll want a recognizable name as Marian, though that's a tough role to sing.
"Some people can thrive and bloom living life in a living room, that's perfect for some people of one hundred and five. But I at least gotta try, when I think of all the sights that I gotta see, all the places I gotta play, all the things that I gotta be at"