A lot of people complain about the AI not developing planets well. I hadn't really noticed, but I looked around in my most recent games and sure enough...
Now, I haven't done any real testing (since I'm not really sure how to), but it would APPEAR that the AI will not build on Terraformed tiles! All the races in my current game (on Painful) have free space on their (otherwise expertly developed) planets, and that free space is about what I'd expect you to get by Terraforming. The races with Habitat Improvement but no Terraforming have fewer undeveloped tiles, even!
Four possible reasons for this occur to me:
1) The tile improvements themselves function as buildings (fact), so maybe the AI regards those tiles as being "already built on" as a result. This could be tested by trading Xeno Ethics to a neutral race before they have a chance to reasearch the Soil Enhancement line, as they'd get those tiles without imagining something on them already. I haven't tested this, though, as it'd take a while to do so, especially since the AI will usually go for Terraforming long before Xeno Ethics (despite being neutral!).
2) Perhaps the AI asseses a planet based on how it is when the AI arrives. I tile that's unusable once may be unusable always.
3) Something else I haven't thought of.
4) A different cause for the problem entirely, me being totally wrong.
I'm leaning towards #1, as it would explain why I sometimes see Arceans with well-developed worlds throughout the game.