Gliese 581c brings up the tidal lock issue: if Gliese 581c is tidally locked to Gliese 581 then one side of the planet would be scorched while the other would be frozen, rendering the planet uninhabitable for water-dependant life. Most of the planets detected so far (including Gliese 581c) have been only detected through indirect measurements of their mass, we no nothing of the structure of that mass. It is possible at least some of these are double planets. Though perhaps unlikely — if Gliese 581c is a double planet, both planets might be tidally lock to each other instead of the star similar to the situation in Asimov's Nemesis, allowing for liquid water and possibly water-dependant life.

The possibility of life on Gliese 581c brings up something that has been a bit of a minor mental obsession of mine: life that has evolved vision is likely to evolve vision keyed to the spectra of light it most often encounters. Life evolving around a red dwarf would not likely see the same spectra of light we see. Blue would be of little use and their visual range would probably be shifted into the infrared. Something printed red-on-white or blue-on-black in our eyes might simply look like a blank page to such a life form and their chosen inks may be only visible in the near-infrared and be invisible to us. Perceptual differences such as that would serve to further complicate communications with intelligent extraterrestrial life. It is easy to assume our perception of light and sound and smell and feel is the default, because within humanity we consider anything outside of that default a disorder. Our bias is sample bias, and other intelligent life in the universe is likely to perceive things in vastly different ways than we can even imagine.