Yup, Divine Fervor is OP, hopefully they won't take too long to nerf it. I reached Legend with a green/red control deck, removing all threats, starving opponent and than flooding board myself. Was pretty fun to grind the ladder, too bad there is no reward for going Legend. Rank 3 unlocks the last card copy and that's it, not even a title. Been testing arena in solo mode, seems to yield more (2 packs average) and 100gold since AI is dumb. In versus arena you can get unlucky and go 1-3 or 2-3 and then get bad rewards.
Legend is the highest rank. When most people make it to Legend they start play-testing instead of using the decks they know are competitive. They already made the top grouping, now it is time to have fun.
Aggro (tempo) decks are competitive, but they aren't OP to the degree you seem to think. I've seen a few of the "serious" legend players wrecking with slower control-oriented decks. Blue/Yellow and Purple/Yellow are the best pairings I've seen. The reason you don't see those as often is due to the cost of the cards. The deck ends up being mostly Epics and Legends, but they definitely do work.