Hearthpwn

I've been running him in some of my druid decks. He is not bad, it's just that he suffers from the same thing every big minion does, being a dead card and not coming out fast enough. If you are playing ramp druid then adding tyrantus is good. Your going to lose to aggro anyway, so why not add this big guy. If your not facing an aggro deck but rather a more midrange/control deck then he is pretty good. If the board has been going back and forth then dropping tyrantus is scary. He can trade 3 for 1 in some cases and 12 damage means if you have a clear shot (a.k.a no taunts) then he will finished the game in 3 turns by himself (assuming your opponent is above 24 hp). The no target effect on a 12/12 body makes him something that is impossible t ignore unless your really behind on board/life total.

I think that we can only hope that the druid quest helps in making a card like this viable. Maybe they're are trying to push a beast/ramp druid deck, and as a result the quest will be something like a permanent mana cost reduction (may like a permanent naga sea witch effect, idk just throwing an idea out). But yes you are right, as of right now this card just doesn't make the cut. 

It really isn't a good card. Hearthstone has evolved a lot over the years and constructed play is now a finely tuned machine. The reason you only ever see a handful of top tier decks is because some thing are obviously better than other and the community quickly learns what does and doesn't work. You are making a lot of assumptions in your reasoning. First you cannot play this on turn 7 and simply Ravage Roar next turn. This card takes all available mana crystals and converts them into 2/2s. This mean that if you play this on turn 7 you fill you board but only have 1 mana  crystal available next turn. If you want to Savage Roar the following turn you also need an available innervate. So this is a combo that requires 3 prerequisites to work: 1. you opponent doesn't clear your board; 2. You draw into Innervate, Living Mana, and Savage Roar; 3. you haven't had to use any of these cards to stay alive up until this turn. This card has so many counters/downsides that it is hard to see it as anything but terrible.

In my opinion this card is just terrible. After thinking about the multiple situations you can find yourself in I have come to the conclusion that this card is usually going to be bad. First lets ignore that obvious counters (e.g. mass dispell, devolve, frost nova) and just talk about the normal situations this card will play into. 10/10 for 5 is really good, but it's not crazy/unheard of, not in a world where jades exist and shaman's have 4 mana 7/7s in a single body. Secondly, you are most likely going to play this in a token druid deck, meaning you are most likely going to play this when you already have stuff on the board, and this is a bad thing. If you already have 3 to 4 minions in play then you will only summon 3 to 4 treants, and at that point this is just and terrible force of nature. Lastly If you play this on an empty or mostly empty board it is ok, but only if you have enough mana left over for savage roar and/or you are comfortably ahead in terms of life total. If you are behind on board and/or on life this card will get you killed. You spend most if not all of your turn playing this card, which has no immediate effect, and if your opponent decides to ignore the treants you are forced to trade into your opponents minions, which means you will get your mana back but you are still forced to skip your turn (the treants give you EMPTY mana crystals). So if you are behind this card effectively makes you skip your turn and loss all initiative.