10 episodes wouldve been perfect had they kept the Left behind DLC, kansas Karen subplot, the long drawn out intro with sarah and Bill's story out of the game. HBO actually ordered 10 episodes but they merged the first two because they didnt feel the first episode was interesting enough.
There are just too many new things added to the show that came at the expense of crucial character development moments for not just Ellie and Joel, but also Sam, Henry, David, Tess and even Bill. Then there is the fact that every scene they cut had Joel and Ellie in it. David gets his own character development with several scenes with his cult, but in the game, you exclusively saw him with Ellie which not just developed his character but also Ellie's. Samething with Sam and Henry. In the game, Henry and Joel have a very adversarial relationship that develops both characters but in the game, Sam and Henry are alone for the first half of the episode. In the game, you spend almost 2 hours with them. Bill's interactions with Ellie help us finally see Ellie come out of her shell and show us for the kind of witty, ballsy, and tough chick under that sweet charming doey eyed exterior.
The show skips all of this to focus almost exclusively on side characters who die at the end of each episode leaving both Joel and Ellie half as developed as they were in the game. Which is why people think the show needed ten more episodes.