January Jones looks the part of Emma Frost, but her particular form of non-acting was definitely off. The cold, aloof Emma of First Class was definitely more in line with her older appearances than who she's become in the comics these days, but it still missed the target. Emma Frost is not a bland henchwoman who fetches ice for Sebastian Shaw. It worked fine for me in this one, with most of the focus on Erik, Charles, and Raven, but if they bother bringing her back, I'd expect it to be atleast a bit larger role. Either recast or force Jones to get her shit together for a sequel, please. I've not seen her in anything else to know if she's always like this or not.
Riptide was pretty pointless except as a prop, I'd have preferred him replaced by Selene. Even if she served the same purpose as him, without any lines, it would have atleast been a bit more Hellfire Club. I feel like they missed out by not having any Azazel/Mystique interaction in this one, maybe next time.
There's actually two awesome cameos, though the second one isn't hard to miss. I liked that one more, as a subtle nod to the original trilogy.