I think alot of people have seen what such mage-like classes do in other games. And with Mesmer i just feel like its one massive CC-fest raining down on you and your friends. And such classes have always been very strong.
Atleast, if i had to call 1 class OP based on what little information we have my vote is for Mesmer. Just, a hunch ofcourse, but based on what ive seen i think itll be a very frustrating class to play against.