The most athletic kid on your team will most likely NOT be the best choice for GK. Athleticism will not make up for intelligence. If a GK isn't smart enough to know where they should be position-wise, distributing, catching, etc. etc. then they will get burned repeatedly. Their athletic talent will only carry them so far. And as stated above, mental toughness AND physical toughness are a pre-requisite. Mentally tough to handle the highs and lows. Physically tough to handle getting trampled on..hands stepped on..kicked in the face..slammed into by multiple players.
My G03 knows she's not the most athletic one out there. But her mental toughness and physical toughness is beyond reproach. One team she played on years ago..the coach told the girls to go out and run until they were spent. They all went out sprinting for about 1.5 miles and then dropped. While they were resting and he was talking to them he realized she wasn't there. She was still out running a decent pace working past her 10th lap when he told her to stop. She asked him why he wanted her to stop..she said she wasn't done yet. She's a diesel not a Ferrari.
Earlier this year she trained for over a week on a broken ankle. Didn't know it was broken. She just thought it was from a nagging ankle sprain from last year and just chalked it up to growing pains and the healing process dragging on. The day she came home from school saying "it hurts a bit" it was swollen HUGE!! I asked how long it's been like this and she said about a week. She just kept icing/Motrin and pushing thru.
And that my friends is what sets GKs apart. Some parents can say their kid is tough..GK parents have real proof. My .02