I think I've given the wrong impression here. Everyone is disagreeing, and probably rightly for how I've phrased it, so let me try again.

I'm not saying that I believe our employee was lazy because he refused to work Sundays. I feel that we hired someone who wasn't sold on the whole startup thing, and treated an early stage startup as if it were a regular job.

I recall a talk from an early Googler, who worked 100 hour weeks for the first five years. Startups need dedication, not because of some kind of culture, or because some exec says so, but because everything is constantly falling apart and it always needs work. You could surely do them differently (perhaps a low-stress startup), but that is another animal altogether.

So the point is you need someone intrinsically motivated to make the startup succeed. Anyone else is the wrong fit.