As weird as it sounds, I think this movie really gained from having less mutants. The only First Class mutants that had a signifcant part were Xavier, Magneto, Mystique, Wolverine, and Beast, but the character development only really focused on the first three. I feel like this made things much more impactful than if they tried to give everyone a bit of something.
The future scenes were well used in support of the main 70's plot and the ticking clock aspect really added some tension to the climax. I think this is the first movie where they made Storm's powers actually badass.
I really liked the ending where they gave everyone a much more natural cameo than I was expecting. To me, it didn't feel like they just through things in for the hell of it.