Looking at how arena is going to shape up, I think Mage will continue to be the unquestioned king. Most important thing is the new common cards, rares and up don't have a very big impact over the long run.
#1 - Mage: Snowchugger, Flamecannon.
Both very solid cards that fit into any deck.
#2 - Shaman: Zapomatic, Crackle.
Very similar to the new mage cards. Quality 2 drop and removal. Hard to say which pair is better, but Mage was a bit better going in so Shaman stays #2.
#3 - Priest: Shrinkmeister, Velen's Chosen.
Even as a naked 2 drop shrinkmeister can fill in fine, but you're likely to get some favorable trades with it even if you don't draft any shadow madness/cabal shadow priest. Velen's Chosen is just spectacular if you get anything to stick. I think Priest will move ahead of rogue and druid with these new early game options.
#4 - Rogue: Goblin Auto-Barber, Tinker's Sharpsword Oil.
These are both board control gaining cards, which rogue was already the master of, but they're solid and now you're guaranteed to get something to wipe the board early even with a poor draft.
#5 - Paladin: Shielded Minibot, Seal of Light.
I have a lower opinion of arena Paladin than most, but these cards might change my mind. Early board control is always a struggle. And vital. These help out a lot, especially the minibot. Seal of light is more of a desperation removal, but at least it's something they were lacking.
#6 - Druid: Anodized Robo-Cub, Druid of the Fang.
I think Druid is going to drop a good bit as neither of these are spectacular. The robo cub is just an alright 2 drop, maybe the taunt can save some games, but it doesn't do anything particularly special. Druid of the Fang will either work spectacularly or sit in your hand and fail spectacularly. I don't like relying on having a board presence as druid and drafting quality beasts (Druid of the Claw excluded) is always a question mark. The class is strong enough already to do fine though.
#7 - Hunter: Cobra Shot, Glaivezooka.
Two of the weaker cards they got this expansion. I admit to being a fairly poor hunter player, so I'm likely underrating them, but I don't think these cards help much. Glaivezooka could be good for taking the board early, but you really want to either have a 1 drop first or a coin 2 drop, which is rolling the dice. Cobra shot just seems terrible for arena unless you're way ahead already.
#8 - Warlock: Darkbomb, Floating Watcher.
The better of the two trash tier arena classes. Darkbomb is solid removal, but Floating Watcher is going to have a lot of trouble here where a 2 drop and hero power can take it out any time before turn 6 or 7. The class needed some amazing cards to move up though.
#9 - Warrior: Ogre Warmaul, Warbot.
Haha. Poor warrior. Weakest class going in and gets the worst card and possibly the second worst, I'm not sure between Cobra Shot and Warbot. Warmaul would be the worst card for any class, but warrior needs fiery waraxe and death's bite just to compete, there's no room to even play this terrible weapon.