1.The last two movies were major disapointments because of this people may think this movie will be no different.
2.The source material is poor for this movie ie Jean and Cyclops are not present in this movie but they were part of the original team in the comics also Havok is in this movie and he should't because he is Cyclops little brother and Cyclops isn't even alive in this movie. These changes will upset fans of the comics and might force them to boycot this movie.
3.Continuity is a major problem in this movie and the film series overall now I now Brian Singer said that this movie ignores origins but it still does't change the issues with the last stand, the very beginning of the movie Xavier and Eric go Jeans house but this takes place in the 80's and there still friends and its said they will become enemies at the end of First Class, this will confuse fans of the comics and the films in alike in general.
4.FOX(thanks nucro for the reminder)is one of the major problems even if people like the look of this movie(and im sure most of you do)they might worry that this movie will be the same as there other comic book movies eg.daredvil,both fantastic four movies the last two x men movies and so fourth and lets be honest all those movies suck,so people might be afraid to see this movie becuse of there other cbm.