I was going to wait before answering, maybe see if someone else wanted to jump in first, but something finally hit me about the OS Issue quad and now I want to share it.
At first, I wasn't sure about any of them. I was looking at all four for examples because all four can end up being illustrated in the story and I was looking for the strongest. And while I was leaning toward skill and experience, I felt like I was missing something but didn't know what.
On the surface level, the lack of skill is the first thing you notice about the humans, and you notice all the robots with their specific skills, and I could see experience through the things I mentioned, but there was still something there that I wasn't seeing. So I tried to remember the movie as best I could and it finally hit me.
The humans are really like a bunch of babies in a hospital all lined up in cribs with nurses taking care of them. Only instead of cribs, it's those constantly moving floating chairs. And instead of nurses, it's the robots. And I guess instead of baby bottles, it's lunch in a cup or whatever the Axiom suggests not long after Wall-E arrives. And while the humans have problems from their lack of skills, just as a baby would, it's not seeing the lack of skills that feels like a problem or that makes the story. When you watch a baby growing up, you see them developing skills, and that's a great part of the story, but it's really about seeing them experience the world for the first time.
And in the same way, I think that's what Wall-E was going for with the adults. I may be stretching the analogy there, that may not be what the writers were thinking at all, but the big moments feel like when the humans have a new experience. That's when the animation really starts to show itself off in a movie that already has some pretty great animation. It's when the music picks up and gets all inspirational. And it's not just for the humans. Wall-E experiences the beauty of space with the same beautiful animation and music.
I'm definitely liking experience as the thematic issue.