Andy Swan tells an insightful tale of how constraints can help drive innovation.
The tricky thing, of course, is that it needs to be just the right constraint... but it is a well-known fact amongst "commercial artists", such as architects, and web developers alike, that "constraints are liberating". Remove all constrains, for example by giving a building an unlimited budget, and you generally create a monstrosity.
Luckily for us entrepreneurs, most startups have draconian constraints. Still, it can be a useful thought experiment to imagine some extra constraints (particularly time constraints) and see what comes out.