Where to start. The acting is terrible (apart from the guy who plays Mat, and rumours are he won't be returning for the second series), the dialogue is awful and everyone delivers their lines with the same broody whisper. The CGI is not great, apart from the first episode, after which it mysteriously falls off a cliff. The pacing is abysmal e.g. it spends almost a whole episode on the death of some incidental character who was had only just showed up. It misses out crucial details and immediately deviates from the books which means that some of the scenes and motivations of the characters just don't make sense. They messed with the magic system, robbing us of the complexity that gets explained over several books so you feel like you are learning along with the characters as they grow.
Honestly, I don't know how they come back from this.