I have learned a lot about how planet habitability works (and doesn't work) in Stellaris in this thread. So thanks to those of you who have paid more attention to planet descriptors and the like than I have.
I have to say, though, that the more I have learned, the less I actually like the habitability system in Stellaris. All the justifications I've heard for why Gaia planets make sense in the context of Stellaris, have only made me disappointed in the lack of variety the game presents. Which is kind of sad for me.
One of the things I like about space games is how diverse life can theoretically be. It's strange that Paradox went through so much trouble to represent some pretty interesting and diverse visual appearances without actually making them very biologically diverse. I sincerely hope this changes over time, because things like 'every world is oxygen-nitrogen atmosphere' honestly just disappoints me, and almost seems worse than Gaia worlds not making sense in a galaxy full of diverse atmosphere. Okay, Gaia worlds make more sense (though I still think they contradict other things), but ultimately at the cost of the galaxy seeming very simple and samey.
Obviously that is a very subjective thing. I'm not trying to say it's not a valid way of doing things.