Margaery would've been one of those younger characters that didn't need the age treatment I think. Not with what is given in the books at least, but she's already grown out of this now, including certain spoilers to details that become quite important later on. I'll have to wait and see, they seem to intend a bigger role for her in the show, maybe she'll even replace the Queen of Thorns?
Brienne was excellent, tackling Loras to the ground did a great job as a first impression, without the long description of the entire mêlée the books had. Also nice to use perspective to emphasise her physical height.
Nice to bring out Theon's inner conflict, the books barely imply this and he's vilified for betraying the Starks. Balon's "plan" to conquer the North is ridiculous of course, and delusional even for him. The North has never been conquered and cannot be held, but that's one of those minor nitpickings
Condensing the events up to Arya's capture was well enough handled. An early introduction of Ser Amory Lorch was probably necessary since the actor playing Gregor Clegane quit, and there probably isn't enough screentime to show the Mountain and his men as the vilest foragers the Lannisters have. Freeing the prisoners was a bit too weak, there wasn't this sence of mortal danger or the fear of those monsters on the other hand.