Hahaha!! This is currently at 28.8% Meta-Defining voting!
Mecha'thun is currently at 28.9% Meta-Defining.
WTF are people doing with their votes!!! JEEZ!!! This must be the "Casual" meta voting LOL!
Well, meta-defining doesn't mean thats card is above "very good". It means it can affect the meta by, for example, creating new archetypes or reviving old ones. Mecha C'thun created some decks builded around him (even if they weren't popular), same with Shudderwock. I'm sure there will be created (and played) Cultist/Ra decks (even if just for fun). HoE members could also count as meta-defining cards, because they try to revive highlander decks, but those cards are pretty bad and not worthy to build around them (maybe except Finley) and without additional support they won't work.