Let's talk Arena now that all cards are out.
Winners:
Druid - THE big winner. Holy shit. Living Roots is an insane common, Druid of the Saber is top, top tier as well because of the flexibility. The 4/4 is just solid and will enable you to actually draft with a beast-focus.
Then we have the rares, where Darnassus is freaking amazing and Savage Combatant will pull some serious weight.
Rogue - Not completely sure about this, but the commons are really strong, all three. The Buccaneer is a great combo-piece and having a 2/2-hero power is well worth the card. Shado-Pan is just pure value. The rares lack some oomph, but Burgle is card-draw so there's that. Cutpurse is probably shit.
Shaman - This is a winner just because of the commons. They're all great, great cards. Ancestrals Knowledge costing 2+2 is way less of an issue in Arena, and getting a 3/4 out on turn 2 is just disgusting. Rares are meh.
Meh:
Mage - Decent commons, good rares. I'm not sure the commons boost the card pool in any significant way though. They might actually worsen it, albeit slightly.
Hunter - King's Elekk is awesome. Auto-pick everytime. Same goes for Powershot. Bear Trap gives you good value, but it's nothing amazing. The rest are pretty bad even if I kind of like Ball of Spiders despite the cost.
Losers:
Paladin - The murloc and the spell common are way worse than what is currently being offered to Paladin in the common slot. I think the same goes for Warhorse Trainer, though to a lesser degree. The joust-rares are a big pile of meh.
Priest - No 2-drop, Priest am cry. Two commons are heal-based which is just trash, but Holy Champion might be great. Rares are pretty bad as well.
Warlock - Jesus. No words. There's a demon-Boulderfist I guess. I might underestimate Wrathguards potential, but 3 health on a 2-drop just doesn't scream "OP!" and the drawback is real. Demonfuse lol.
Warrior - #ArenaWarriorsMatter