That super dread infernal is nutso.. It's a fire elemental at worst, which is still fine against control. Most control decks still play minions throughout the course of a game.
If all you do is kill a 3/3 with it, great. If you clear their board while playing a 6/6, even better.
It may not see a ton of play for other reasons, midrange/control warlock not being positioned well for example, but it's still a great card. It'll definitely crush in arena, holy shit. Warlock would be #1 or #2 class with Priest if the expansion released today with no more cards. That's arguably better then Firelands Portal for arena, but slightly different so hard to compare directly. If your opponent has at least 2 minions out, it's always better unless you cant take 3 damage. With just 1 minion, it's about the same. A 6/6 and a hellfire should be about 9.5 mana, so you are saving 2.5 mana, roughly.
That Mage AoE is cool, but totally expected to me. There was sort of a void at just 'deal 2' AoE between all the classes. Not talking about AoE's with an extra effect, like Blizzard, or conditional like the murloc. Had to figure one of them would get a basic 'deal 2 to all' spell at some point. We had deal 1's for 2 mana, deal 3's for 4-5 mana, deal 4 at 7, but sort of skipped a deal 2 for 3-4 mana. Half a flamestrike seems most at home in mage, thematically, but for balancing purposes it could have gone to a few classes. Though, after they gave shaman a strictly better arcane explosion, guess they had to throw it back to mage.
Those hog rider guys.. idk, seems weak. The big one would be best, since bigger stats + charge is great, but how often does your opponent have 6+ cards in hand after turn 5? I suppose against a control deck it's great, but seems risky to put in your deck then. If you play against aggro/midrange, it's a pretty bad card. In arena, the 4 drop is playable. It's not horrible to play a 4/4 on turn 4 if you have no other option, but if you hold it as a 'removal' spell in the late game after dumping your hand, then it's great. Also a great topdeck after both players dump their hand.