Besides a lot of unnecessary changes that got rid of iconic moments from the game (or changed them to be less impactful or awkward), they diluted Ellie down heavily. In the game she's cold, calculated, and ruthless. In the show she's emotional. I think they tried to keep more of the old Ellie's personality.

Aside from that, Tommy's role from the game is completely erased, which was a massive eye-opening context for pre-TLOU Joel, especially for Dina. Not that it's new information as much as witnessing it.

It's just disappointing because they did such a good job with season 1. And Bella Ramsey has gotten a lot of flak for bad writing.