I'm not really enjoying the retcon (spoilers)

... that Vecna is the one who shaped the appearance of the Upside Down, controls the creatures who live there, and has been inexplicably operating behind-the-scenes the entire time.

If there had been some kind of foreshadowing to Vecna's existence at the beginning of the show, I think that would have made it more palatable. But having the Demogorgon be revealed as a footsoldier of the Mind Flayer, who is then revealed as merely a creation of Vecna, feels like a series of diminishing returns. It reminds me of 'Lost', which was clearly being written from season to season, without a long-term roadmap.

There was also a simplicity and mystery to the Upside Down and the Demogorgon in Season 1. Vecna's Upside Down is just way too packed with backstory, clocks, bats, tendrils and his personal vendetta against 11.