Bast casting
Having read the book, Igraine is around 19 when she first meets Uthur but in the tv series she looks mid/late thirties at best(and a haggard one at that) in the show. She's also described as very beautiful but the actress is matronly and a little homely. Same with Uthur - pale blond hair, tall and lanky and a sarcastic amused character but in the show he's short and stocky, dark haired and serious.
I just think, in a book like this where the characters of the fantasy plot are what move the story forward, things like that are important. Especially the time period its set in, even Gwen looks like she's in her 30s (she is also a terrible actress) but women married young (clearly I wouldn't expect 15 years old playing the role but actresses who looked youthful enough to pull it off).
The show is a disappointing execution of the book