It's bloody difficult picking an all time great 15. It's a bit like picking the greatest rock songs of all time. It gets harder every year as more great players emerge. If you picked that team 20 years ago half those players wouldn't be available. It's probably easier to pick the top five in each position. Being a player like Fitzy you'd think he could pick a team based on players he played with or against.
Maa Nonu is an interesting choice. Once upon a time nobody would have been ahead of Tim Horan in my team. Hmm
As CI said, probably chosen for his UK audience hence the lack of Wallabies and Boks.