Trade only: Yes, you go first
They can forfeit the limitation of always having a balanced amount of cards per class and rarity in the mini-sets because they smaller size. This allows them to print new cards to power-up already existing archetypes, push new archetypes in certain classes and also print more powerful cards with more interesting effects. The previous way of designing legendaries for smaller sets/adventures required that they always be neutral (in order to not upset the balance in the amount of legendaries in classes has at their disposal). In my opinion, this is a silly limitation and having more flexibilty allows Blizzard to print more compelling cards design-wise.
The real cost is actually 3 mana. Troll Trump.
We already have two professors confirmed from the announcement video: Speaker Gidra, professor of nature & the elements, pretty obvious Druid/Shaman combo and Keymaster Alabaster, professor of replication, who is a trickier guess but I would say either rogue/mage (as both classes have cards that make illusions or copy minions) or priest/warlock (if they go the deathrattle/cloning route like Boomsday).
Mildly contaminated pally.
I think it is somewhat skill-testing (unlike most of the other demon hunter cards). Really strong with the summon 6 Illidaris with rush. But I have seen it played very suboptimally. It is also very fragile at only 1 HP. There are more broken cards in the DH arsenal for sure (Antean, Skull, Priestess, etc.).
If you can't weather the initial onslaught it's impossible to win. Even if you clear and their board and come back they refill it very quickly. They also have very good burst damage so if you can't set up some huge taunts they will kill you over the board with Metamorphosis, Priestesses or the +8 damage thing. Some classes, like druid or priest, are well positioned against the aggresive variations of DH. Most other decks just can't keep up. Their tempo is the best in game right now. I did had some success with Libram Paladin (the deck recipe list) but I think my opponents missplayed a lot (probably didn't knew how the new cards worked exactly). Their 1 and 2 drops are insane. 2/2 for 1 that buffed everytime you attack, Umberwing weapon and rushing murloc. You also can't really outrace them because they can heal with eye beam or the new taz'dingo with lifesteal. They late game is scary too, priestess and Antean just push so much damage and clear boards constantly. They draw is also insane (Skull of Gul'dan is just broken) so they never run out of resources...
All in all I expect big nerfs all around. Probably in the coming weeks. The game will devolve to just DH vs DH or DH vs Druid/Priest. Others classes are practically deleted from the game. They pushed the class too hard.
145 packs. 12 legendaries (2 golden). 32 epics (1 golden). All rares (11 golden). All commons (13 golden).
Legends were Zixor, Apex Predator, Beastmaster Leoroxx, Evocation, Astromancer Solarian, Murgur Murgurgle, Lady Liadrin, Reliquary of Souls, Soul Mirror (golden), Shadowjeweler Hanar, Akama, Lady Vashj and Al'ar (also golden).