I think an important open question is how much gravity is enough. Will Mars's 0.38g be enough for colonists? How about the Moon's 0.165g? These are critical questions we need to answer to know how practical it will be to have colonies on those worlds. The only choice my be to breed humans for those conditions. Homo ares and Homo luna.

Venus is such a tragedy. Surface gravity of 0.9g, probably quite livable, but the surface is Hell.