Pinky: "Gee, Brain, what do you want to do tonight?"
The Brain: "The same thing we do every night, Pinky — try to take over the world!"
My memory is a bit fuzzy, but from what I remember, if the Brain had set his sights slightly lower, he definitely could have taken over a city, or perhaps a small state as the first step in one night, and left the rest of the world to following nights.
Along these lines, I was talking with Dan about why I thought of Stack Overflow/Exchange as being significantly more successful than Quora. I wouldn't be surprised to find out that they have comparable traffic, users, or other metrics. However, from an outsider's perspective, Stack Overflow made fast progress on its initial goal of being a good programming Q&A site. There was never a clear mission accomplished moment, but at this point its success does not feel in doubt. There were follow-on steps, some more successful than others, and a general upward-and-onward feeling.
On the other hand, Quora's goals from the start were outrageous (in a good way): “Imagine a world where I knew everything that I wanted to know, as long as someone else in the world knew it.” I'm sure that having J.J. Abrams give his thoughts on monster/action scenes is a milemarker on that path. However, it's harder to see how far they've come or to feel like the site has a well-functioning foundation/core functionality, since the path is a continuous curve rather than a step function.*
Google might be considered a counter-example to this; from very early on its goal was quite broad and audacious. However, having a steady stream of corpora to add shows definite progress. There is also the matter of perceived goals versus actual internal goals.
Thefacebook was long discounted by some as being a site just for college kids, surely even after they set their sights higher. Having others underestimate your ambition (but not too much, lest they ignore you) seems beneficial.