Some have already quoted this "The Avengers film of the X-men world" which I suppose is fair for such a massive undertaking. The scope of this film is mind boggling. The film is an attempt to bring the success of the previous X-Men First Class in line with the two original X-men films. It would have been much easier to simply abandon the original star studded cast and continue with the First Class Cast. However with the success of the Wolverine film and Bryan Singer eager to reclaim his baby we have X-men Days of Future Past.
This works out neatly with solving some of the problems of bringing the old franchise and the new franchise together. The story Days of Future Past is actually one of the better written comic series (voted #25 in Marvel's Greatest Comic of all time.) Alright we have a good start! Time travel fixes most problems, if only I had a time machine I could stop X-men 3, X-men Origins: Wolverine, and Superman Returns (I still get night terrors from that one.) But how can we fix all the issues from the previous films which I shall list now: Professor Xavier dies in X-3, Professor Xavier is walking reunited with Magneto at the start of X-3, so many side-characters are killed off in Wolverine: X-men Origins, Cyclops is killed off in X-3, Saber tooth goes from being an intelligent brother of Wolverine to a near mindless brute, and Professor Xavier is walking in X-men Origins again. Answering so many questions is like watching Lost, every time you answer a question another one will appear in it's place. My expectation of this film is re-write some of X-men history. Sure I love it when things go neatly together as some massive circle but realistically with the damage done in X-men Origins alone there is no way we can give this film justice. Fans want to see the mutants we love, Cyclops needs to have a bigger role (he was team leader after all), Iceman needs to be the strong mutant he was penned to be, Wolverine doesn't have to be the center of all attention (though as it appears he once again is the time traveling center of the universe- however it makes sense since he is a constant- replacing Kitty Pryde as the time traveler), Sentinels need to be a bigger part of the movie, and freaking Gambit needs to be a bigger role (rather than the 3 seconds devoted to his unnecessary character in Origins) as well as recast (preferably by Josh Holloway.) Realistically I know all these requests will not come to past, but like the trailer says "please we need you to hope again" and I believe they can recapture the magic that was X-men.
And here is a picture of Bryan Singer (on the right) that he he tweeted, showcasing the Sentinels.