Elizabeth Kolbert makes a point I'd never considered about space colonization as a strategy for species survival, one that also provides a neat solution to the Fermi paradox:
[W]e’ve had the capacity to blow ourselves to smithereens. One of these days, we may well do ourselves in; certainly we’re already killing off a whole lot of other species. But the problem with thinking of Mars as a fallback planet (besides the lack of oxygen and air pressure and food and liquid water) is that it overlooks the obvious. Wherever we go, we’ll take ourselves with us. Either we’re capable of dealing with the challenges posed by our own intelligence or we’re not. Perhaps the reason we haven’t met any alien beings is that those which survive aren’t the type to go zipping around the galaxy.
And remember: No matter what mistakes you've made in your life, you'll probably never make a mistake as consequential as the Lockheed Martin software programmer who, in preparation for the Mars Climate Orbiter project in 1998, "neglected to convert English units into metric ones."
UPDATE (6.6.15): Fun fact from Wikipedia about the metrication fail: "The discrepancy between calculated and measured position, resulting in the discrepancy between desired and actual orbit insertion altitude, had been noticed earlier by at least two navigators, whose concerns were dismissed."