a) The scene's execution on screen was very poor, such as Dany mastering Drogon off camera, as pointed out by someone else already. It was a massive contrast to every other scene in the episode that was wonderful staging, storytelling, directing, dialogue, etc. As the ending scene it was a let down on the excellent work that went before it.
b) We had one of these Dany conqueror endings just 2 episodes ago. The circumstances have only changed slightly. So boring. I'm not a big Daenerys fan anyway, but I can appreciate her triumphal moments...in moderation. This was just so same samey, and unless you're super invested in her story, it's dull. We get it, we get it already.
c) There were a plethora of other scenes that would have been better for the episode's climax. I thought we were going to end on Benjen, which we'd been made to wait 40 mins for from the first scene, and 6 seasons before that for confirmation. That was the big reveal in the episode, and a better climactic moment than the return of Drogon. It was another Stark reunion, for goodness sake! It's gravity outweighed yet another Dany speech, which we're so used to already. Many viewers would have appreciated another couple of minutes with Benjen.
d) I think people are also disappointed in the depiction of the Dothraki themselves too. They cheered for Dany in such a monolithic, one-dimensional fashion. Kind of cheesy in fact. This is the same criticism levied at the Iron Islanders and the Kingsmoot last episode.
These are all just my interpretations, but I hope to have pointed out that disliking Daenerys is not the only reason people justifiably disliked the final scene of the episode.