Usually I would agree, but not in this instance. Unlike cards like BGH, Harrison, Loatheb and TBK, Emperor's ability is not one that is meta dependent or difficult to evaluate. All of the four previously mentioned cards come in and out of favor depending on the meta. BGH was much less prevalent before GvG, Harrison was almost never played at the height of Naxx, Loatheb was ridiculously strong when combo decks like Miracle Rogue were still prevalent right after Naxx's release and TBK comes in and out of the meta with the prevalence of Druid.

I think Thaurissan is much more comparable to Sylvanas in terms of his guaranteed value and power level, although I personally feel he's considerably more powerful. Thaurissan is a card that you can drop and get value out of no matter what deck your opponent is playing, and the value isn't reliant on what cards you're running at all. Cards that allow you to play other cards sooner/easier have consistently been among the strongest in a set throughout every major card game, from Magic to YuGiOh. It's less prevalent in Hearthstone because prior to Thaurissan, every other card with a similar effect was either really niche or too hard to have the effect go off. The best cards to compare Thaurissan to in this area are Mechwarper and Sorcerer's Apprentice. Both of these cards see play in pretty much every deck that uses a lot of cards that they reduce. Almost all tempo mage decks play Apprentice, (although freeze mage doesn't because the way they use spells is different), and every mech deck runs mechwarper (a card that was considered incredibly OP just one month ago). What sets Thaurissan apart from even these, though, is that he doesn't need to remain on the field for his mana reduction to be utilized and he affects every card in the game.

A 5/5 is a decent body and even if he gets no use out of his ability he still doesn't set you behind all that much for 6 mana. This is why Sylvanas is consistently run throughout all the changing metas. (Not to mention there's a considerable lack of solid 6-Drops, although slightly less now with Sky Golems). What makes Thaurissan better is that he ALWAYS gets value unless you're topdecking, which you almost never are in a control deck and rarely are in a midrange. Most cards in this game would be incredibly powerful if they cost one less mana. Turn 10 FoN/Savage Roar/Savage Roar for 22 damage? Turn 8 Fireball/Fireball/Frostbolt/Frostbolt/Ice Lance/Ice Lance for 26? Or even something that's not even a bit unrealistic like coin Thaurissan into turn 6 Boom into turn 7 Ragnaros. The thing about Thaurissan is not just that he reduces the cost of cards, but that unless your opponent also plays him then you essentially get a free coin at the start of each turn. He just gives so much free tempo that he will always be strong. There is always the chance that I could be wrong, but I highly suspect that we will see no change in Thaurissan's power level until he receives a nerf. I expect him to be even more widespread than Dr. Boom.