Density of information. It needs to not only be short, but to convey a lot at the same time. This is why analogies (like "Uber for x") are popular. You need to

Density of information. It needs to not only be short, but to convey a lot at the same time. This is why analogies (like "Uber for x") are popular. You need to capture something of the spirit of the venture, as well as what differentiates it from its competitors.