I get the desire to leave some ambiguity to allow for fan theories and all that but for me personally I feel like the connections and laws regarding all the supernatural elements (Eleven, the Upside-Down, the monster, Will being able to communicate at times etc...) weren't fleshed out nor was there enough background about the government lab. A fictional universe where supernatural stuff happens without clear laws regarding what is possible or not and why in that universe always comes off lazy to me, and I felt that way about this show.
It was entertaining and the kid actors were impressively not annoying, but I didn't think it was that spectacular.