Did you miss the part where there have been two flashback mini-arcs devoted solely to giving characters more backstory, with a third on Isshin planned?
As for character development, Ichigo, Orihime, and Chad got plenty of it in the opening arc, while Chad, Rukia, and Renji got a lot of it during the SS arc. Grimmjow had quite a bit some recently as well (though unfortunately it made him a shittier character for it, much like Renji's).>>13682660
It didn't get rejected in its present form, obviously. They wouldn't have accepted it if it was still as copypasta as it was when he first submitted it.
And on that note, I've heard it was rejected, but I've never heard that it was rejected four times. Source?