Well for one they've at times made Ellie seem a bit thick - Ellie is meant to be clever/resourceful... yes early in game at one point she's saved by Dina, but in the show I swear they're making it seem like she wouldn't last 5 minutes on her own. That part aside... her still having the jokey/upbeat nature of S1 Ellie makes ZERO narrative sense. TLOU2 Ellie is rage-fuelled, depressed, and single-minded and the character HAS to be that because -- objectively speaking -- the Seattle mission is a suicide mission. They're entering a strange city full of soldiers to track down and kill a group of killers. Ellie being unable to see past her rage and trauma is half the plot. But the show has her wisecrackin' and telling jokes like "young Ellie" ... the whole point is that Joel's death broke her ... the Ellie who is in the headspace of the Ellie in the show wouldn't be in Seattle. You could argue that she's masking her trauma but if that's what it's meant to be, it's not selling. The key example I suppose is the "I suck at math and don't know what a protractor is hehe" scene... shocking writing IMO. Like yes it's fine as a throwaway joke. Prolly not when Dina is in the process of
trying to figure out where Joel's killer is in a city full of soldiers 
. It would only make sense if she was more herself/jokey when distracted/when they're just hanging out, before getting back to business. Joking about the actual search? It does not make sense.
And don't get me started on Dina's "I'd track them down forever and never stop" speech like wtf that is going to be a major stumbling block later on, unless they've entirely changed another crucial part of the plot and theme
. Show Dina is also out for revenge. Game Dina is
only there because she loves Ellie. It's a huge and important plot distinction. And [double spoilers] it's going to make Dina look like an
absolute dick if the post-seattle farm/2nd mission/return plays out the same way.
I'm not a stickler for adaptations changing things from original version but with this they're doing something really odd; changing major details like this, whilst NOT changing the overall direction of the plot/major "set piece" moments, and the result doesn't make sense. Like by all means change it but that would have knock on effects.