What is the kiss of death for a startup? I say: the "hard face."
The hard face is like it sounds: a facial expression of seriousness, quite common in seasoned businessmen, politicians, and their ilk. Perhaps such an attitude is unavoidable, and maybe even optimal in these people's environments. But certainly not in the realm of creativity, not the realm of startups.
As you grow older, you will inevitably be tempted by the demon of seriousness. Avoid it! Punish yourself with a little sillyness every once in a while should that demon get to you. (This post, for example, is silly, but is not a punishment.)
While a serious understanding is most definitely needed in startups (like all business), you mustn't let it define your world. Because it will constrict that world. The people I am most impressed with, those I have formed partnerships with, have all been, without an exception, relatively childish.
"Childish" sounds like a bad word, but it is really the best word to describe what I am trying to get at. Such a person lets their thoughts and feelings out. They explore. They are free. They are honest. In short, they are eminently human. And without the constrictions of seriousness, they are exceedingly productive and creative.
The alternative is to become rigid, stagnant, and untrustworthy. This is perhaps the appropriate attitude of a dictator, or someone like Larry Ellison. They've got lots to protect, lots of political games to play, but not much to create or give. Such rigidity is the opposite of creative, of new life. Life is always soft in the beginning. All creation is. And, should we wish to be creators, we must endeavour to make ourselves soft. We can have some hard heads and faces when we need them, but we will never create great and new things without a childlike face, and a childlike smile.