What makes for a good demo ?

Now that the next fest is over, is there anything that stands out that makes for a good metroidvania demo ?

The mood ? The controls ? Many abilities on display (kinda spoiler-y) ? Secrets ? Story tease ?

In a way I think that less is more, in that I dont want to be spoiled before the game is out, but just the begining of the game with like a single boss or a single ability can feel a bit .. dull ?