I'm very excited for this set. I like the new "Mech" type, and it's interesting that existing cards like Harvest Golem are getting the type added to them. I guess being a digital format means that they can do these tweaks that require games like MTG to bring out a whole new card.
Some of the cards that initially grab my attention are Clockwork Giant, Bouncing Blade and Shrinkmeister.
Clockwork Giant is going to be interesting against decks like Handlock, and might bring out some combos with cards like Coldlight Oracle, and Sap/Vanish mechanics etc. New Giants Rogue Meta?
Bouncing Blade is another card that can potentially do a lot of damage if you're facing down a board full of big bodied minions
I think Shrinkmeister is going to be a pain for my Druid deck, I 'm going to be a little more scared for all my 4/x creatures. But I also know how frustrating it is to be the Priest going up against all those 4/x creatures so this is a nice little card.
I think a sleeper card for me is Spider Tank. I don't think there is going to be a lot of hype around this card, but I think it is going to become another Yeti, because apart from Priest we haven't seen another 3/4 creature for 3 mana, which doesn't seem like a big deal, but it fits curve so well.