Dark World was the second outing for Thor. It was criticized for its storyline, that while having some holes, was still very good.
The film revolved around the Dark Elves who were at war with Asgard and released a force called the Aether. They lost however, and Asgard buried the Aether, but then it was discovered by Thor’s love interest Jane Foster (Natalie Prtoman) and entered her body. That led to a new war between the head of the Dark Elves Malekith and Odin (Anthony Hopkins) and the Thunder God Thor (Chris Hemsworth).
There’s an underlying theme of family conflict and reconciliation as well. Loki (Tom Hiddleston) is imprisoned at the start for his attack on Earth that occurred in the first Avengers’ film, but not executed because his mother Frigga (Rene Russo) still loves him. She is Loki’s only real connection with another even though he doesn’t want to admit it. When Frigga is slain by the Dark Elves it starts Loki’s reformation as he’s willing to work with Thor for revenge. This relationship would be further explored in Thor Ragnarok.
I think the main criticism was that there was just too much darkness in the movie. There were not only the Dark Elves, but the dark Aether, and then Malekith turned a black Dark Elf into Darkness to attack Asgard. Seriously though I think a lot of people just felt like the story fell flat. Some of this is warranted. Dark World is not as good as the first Thor movie and nothing compared to Ragnarok. That being said, it’s far better than Captain Marvel, Spider Man Far From Home or even Dr. Strange which are the worse releases by marvel so far. Yeah, there’s a lull in the middle, but when Thor decides to work with Loki things get a lot better all the way to the climax at the end.