Jon Favreau wasn't considered a top talent in any sense when they chose him to direct Iron Man
; he'd directed three movies, only one of which (Elf
) had been any kind of success (though it was a big one), and that was in a completely unrelated genre. And, frankly, his one film post-Iron Man
didn't set the world on fire. Branagh certainly has never worked on this scale before; neither has Whedon, despite being a big name in SF/F circles.
I don't know much about Taylor, but they picked a promising TV director with experience in the sort of medieval fantasy genre that Thor has a lot to do with (based solely on his resume, he's more suited for the film than Jenkins; but obviously, as Favreau and Branagh indicate, that doesn't necessarily mean anything). There's plenty of talent in Hollywood waiting for a crack at directing a major picture.