The first episode was an hour, but eps 2 and 3 were less, around 40-50 mins. (I haven't watched 4-8 yet.) So there was some loss of runtime, and I understand the need to change some things to make up that time. However, (and granted I'm only three eps in) I doesn't feel like the changes that were made were made strictly for runtime reasons.
Gran-Gran giving Katara the scroll instead of her stealing it, yeah, I see that being a time saver. The overall change in Katara's personality? Not so much.
Not an unsecured passenger that's thrown out of the car, an unsecured passenger being thrown into another passenger who is in the car.