I quite like the DA idea of requiring certain techs in order to colonize a hostile planet environment, however, what about conquering such a planet that has already been 'tamed'. Right now, if one conquers a Barren planet, for example, owned by another civ, and don't have the Barren colonization tech, then once conquered, the planet is more or less unusable. This doesn't make much sense to me. If the planet was successfully colonized by a foreign power, and this planet in turn is conquered, short of a destructive invasion, wouldn't it make sense to be able to exploit the planet as it has already undergone the necessary steps to be habitable? I may not have that giant oxygenation machine, but that doesn't mean I can't enjoy the one already there. Of course if a planet were only 50% usable by the previous occupant, I will need to develop both levels of the colonization tech to be able to reach 100%.

An exception to this would be if the previous occupants had special genetic characteristics that meant certain colonization techniques were unnecessary. Ex: the civ is a bunch of underwater dwellers, so they don't need special techs to live on aquatic planets.