I'm gonna say warlocks are definitely one of the bigger changes. Far bigger than hunters.

The basic gameplay for hunters will likely not change very much. Their abilities will probably be mostly unchanged. The difference will be that instead of having a slowly regenerating mana bar, they'll have a quickly regenerating focus bar.

Playing warlocks, on the other hand, will be a totally different experience from the ground up, with an entirely new mechanic designed to be used in every single fight, to boost the power of any number of abilities. This means not only that one of our oldest mechanics is changing dramatically, but that many warlock abilities will now have a secondary special effect to use in conjunction with Soul Burn. Throw on top of this major changes to itemization and the hinted possibility of an entirely new pet, and, well, I'm sure you can see how things might be a bit of a bigger change for us.

Death Knights are also pretty major, as this means the playstyle for one spec will become focused and very different from how it is now, and also means that lots of talents will be shuffled out of other trees, making the options for each tree very different.