Fans either liked Monty and Harper's hook up or they didn't and felt it made no sense for them to suddenly have feelings or be attracted to each other. Some fans also felt that the writers just turned a gay character into a straight character as Monty was considered ambiguously gay by some fans because he didn't show very much interest in women until now and the show mostly focused on his friendship with Jasper even then the hookup has sunk the Jasper/Monty ship.
Whether or not Luna should have taken the Flame at the end as some think she is stupid for not taking it while others are glad she didn't as it would contradict her character by having her give up her pacifist code because of one incident where she had to break it. In fact the incident where she kills A.L.I.E's people is implied to actually show why she doesn't like killing in the first place and she wouldn't want to do it again no matter what.
Though Clarke is technically trying to put Lexa into Luna's head.
Strangled by the Red String: Some fans feel this way about the Harper/Monty hook up as because they were mostly in the background it wasn't really shown them developing feelings for each other.
