The problem with Emperor is that the card will only become more of a problem with time. I honestly believe that the card's effect is quite under-costed, and it has an effect that mostly takes viable combo decks and puts them over the top.
Decks running Emperor need to be reliable without it(since playing Emperor early can not be done reliably), but he is going to continue accelerating combos to unfair turns, and that makes his current form as a 6/5/5 with a guaranteed activation too potent.
I think another way to look at the same phenomenon is to say that many decks which could not possibly be competitive if emperor didn't exist are now competitive, and new decks will spring forward from now on that use him as a lynch pin.
That is why I would rather not nerf emperor: he enables the existence of many decks which otherwise could not feasibly exist. Nerfing something like Warsong Commander or Battle Rage basically only nerfs Patron Warrior and that's it; nerfing Thaurrisan nerfs all sorts of decks, including those that don't exist yet but will with the inclusion of TGT and other expansions.