"All in the Family: Each class belongs to one of the city's three factions. Your class cards and neutral minions all fall into that faction's theme. The affiliations are:
Grimy Goons: Hunter, Paladin, Warrior
Kabal: Mage, Priest, Warlock
Jade Lotus: Druid, Rogue, Shaman
Faction Specials: Aside from class affiliations, the three factions each have their own unique twist.
Multi-Class Cards: Some neutral minions are only available to members of their faction; for example, a Grimestreet Informant (a member of the Grimy Goons) can only be in Hunter, Paladin, or Warrior decks."
I like that this expansion allows you many more uses for the cards in that certain cards can be used by multiple classes. From the cards they've revealed so far the expansion looks to make Priest a much more viable class. Some say Priest is headed for tier 1 status and from the cards they've revealed, that's a solid possibility. So far Blizzard hasn't dropped a release date yet but if they are keeping with the two expansions and an adventure release clip it should be out this month. Currently I have over 6k gold so on Gadgetzan launch I should be able to open over 60 packs! Whereas I won't be able to get every card I will get many of them and opening that many packs is always a lot of fun.
If you are looking to get back in to Hearthstone I recommend you check it out as Blizzard recently added quests that make it much easier to get gold. Many quests are now things like "Play 10 pirates" or "Play 20 battlecry minions" making the game much easier to run through your dailies. You might need to re-roll your quests if you have a full roster of old "Win X Games" quests but if you stepped away because Hearthstone was too hard a grind, it might be more accessible to you now.
Someone on Reddit is updating the link below with all the new Gadgetzan cards so head over there if you are curious what will be coming in the expansion.