My main issue with Arya saving the day was that it was just the same old tactic that had been used a whole bunch of times during that same episode:
1) Character is in mortal danger
2) Thing trying to kill character gets ready for the final blow
3) No wait! Here's other character to save the day!
That they decided to go with
literally just that when it came to the Night King's defeat really deflated things. You knew something like that was going to happen and they had even foreshadowed that Arya herself would do it so when it did happen I didn't feel any kind of exciting payoff. It's just a thing that happened because it 'made sense', and that's not good storytelling in my eyes.
I hate to make a comparison but
this scene from Infinity War did the whole 'Villain slowly makes his way towards the macguffin' thing
much better. Frankly I'd say Thanos in that film felt magnitudes more threatening than the Night King despite having so much less build-up.