I would think that time, temperature and atmosphere (pressure and composition) would be the major considerations. Obviously we are talking light years to travel to some planets and we do not yet have that capability for that. Temperature is a huge problem because in our solar system, planets range in surface temperature from greater than 400 degree celsius (mercury) to -218 degrees celsius (Neptune), and that is just druing the day, the extremes are also severe during the night. The climate is also quite different to Earth, with sulfurinc acid rain, winds, fog and frost on other planets. All in all, a very complex situation to try and work with. So the first problem would undoubtedly be getting there, the second would be protecting ourselves from the extreme conditions once we arrive!