It's gotta be "Seeing In Game footage", no? When it gets down to it, that's the main thing that matters, and its what will show what we'll be playing. Then decide how pumped you are and how much you will Kickstart. All the other ones are interesting, but you know.
Combination of gameplay and the music, but I voted for gameplay. I need to see exactly how it is turning out so that I can stop saying that I am only 99% certain that this game will not disappoint me in any way. Music is an amazing bonus and already has my 100% certainty of being great.
Character and move reveals are something I would like to avoid until I get my hands on the game myself, but it would be hard to avoid that and still see gameplay.