
The first two episodes were literally just fan service and characters talking about how they were going to die. Maybe 5 minutes was dedicated to actual WW screentime, and then in the second episode they do a slow walk up and the NK gets stabbed and that's it.
The Night King's death was too easy and therefore disappointing. It was very much 'oh, that's it?' for a lot of people. They only reached Winterfell before their plans collapsed, despite this being built up since S1 as an apocalyptic event which will tear apart the world of men. Instead, we get one actual battle (hardhome was just a massacre) and then arya does a knife trick .. problem solved.
We learn almost nothing about the NK and his motives, he's just evil for the sake of being evil. He's not fleshed out, his arc ends before it's really begun. The white walker threat even only manages to kill off a few characters, it's all just very meh for something which was built up as huge and world shattering.