Gliese 581
Even though these two planets might be the right temperature for life to live, other issues could make their environments hostile.  Planet 581g might be stuck in an orbit resonance, where the planet spins at the same rate that it orbits around the star.  This would mean that one side of the planet always faces the star, just like the same side of the Moon always faces the Earth.  If this is the case, the side that faces the star would be too hot for people to live, while the other side would be too cold.  Planet 581d might also be caught in an orbit resonance.