Tivadar's Crusade is one of my all-time favorite cards, because I once had a friend that played this all-Goblin (no burn except Goblin Grenade) deck for fun. It was the deck that we would run our silly decks against, so it generally did pretty well. So one time I fished a bunch of Tivadar's Crusades out of a common bin at my local shop, then built an (intentionally) terrible white deck. We played multiplayer, and I pretty much got ignored because I was playing Tundra Wolves and the like. Finally it was just the two of us, I dropped a Tivadar's Crusade (he had never heard of it), and I swarmed over with a bunch of Pikemen and Ication Infantry for the ridiculous win.
That said, I doubt it's the defining card for the set. Any of the current options sound good, though Ball Lightning isn't very casual, and Festival is a slightly different Fog.
...Of course, you haven't lived till you've built an Erosion deck...