The size of the market and obstacles in the way. For example, K-College education in the U.S. is about a $1 trillion a year economy. Government and self-interested trade organizations stands in the way of disruption (they don't want to be disrupted they want to keep their jobs at the expense of everyone else).

I think I have to disagree with your logic. Apple, Microsoft, Facebook and Twitter would not have been considered ambitious. They mostly created their own markets or were so early to a market that had huge potential that they didn't have significant market size or 'obstacles in the way'.

This is a very good point. For example, would Google have been considered ambitious (trying to dislodge the dominant search engines) or conservative (launching into an established market)?