A very "stable" psychologic profile, the ability to deal with information overflow. I've seen seemingly very rational people doing pure nonsense when they were under high stress (like beaching a sailboat straight on a very visible sandbank, after 3 repeated warnings...). The ability to put aside personal feelings so that it won't interfere when the crewmate you don't like asks you to do a specific task. Of course good reflexes and coordination, and excellent spatial orientation (very useful when you're docking a spacecraft with an offset docking port like the Shuttle, and also for limiting the "falling into Earth" feeling/disorientation on EVAs). Well inner ear is something that can be trained, but probably some people are better than others. Also you have to understand how work the systems you're working with, and in the case of the Space Shuttle (1M spare parts...), it was no small feat. So most astronauts are highly specialized (PhD) in space-systems engineering, some have double-specialization for mission specialists skills (like biology, psychology, medicine...). Which means those are people that learn fast and well.