4. Having Loki Constantly Jumping Between Good & Evil
Loki was good for the first half of the first Thor movie, before becoming a villain. Then, in the Avengers, he was the main antagonist, but showed glimpses of his good side (such as when Thor tried to get through to him atop of Stark Tower and he looked remorseful for a moment). In this movie, it is expected that he is to be taken out of his cell (where he is living his life as a punishment for his war crimes in The Avengers) in order for him to help against the forces and scheming of Malekith. Whether or not he will honour his word and remain loyal to Thor and the forces of good remains to be seen, but if he starts off good and then turns on his brother again, it will become very tiresome. Of course, Loki is known to be morally ambiguous in the comics (hovering on the border between good and evil - though mostly being evil), but to do so on such a regular basis over such a relatively short period of time in the MCU would start to get silly. He should be either good or evil in this movie, either sacrificing himself for the greater good or showing himself to be just as evil as he was in The Avengers. It will do absolutely nothing for the viewer's opinions of Thor's intelligence if he trusts Loki and it turns out he's on Malekith's side.