Gadgetzan brings 132 new cards to the game; a motley assortment dredged from the underbelly of Warcraft. Many of those cards are associated with three new factions, themselves associated with specific deck classes in Hearthstone.
Hunter, paladin and warrior decks are associated with the Grimy Goons, a burly bunch. Mage, priest and warlock decks are associated with the mystical Kabal. And druid, rogue and shaman decks are associated with the Jade Lotus assassins.
Each faction has "tri-class" cards usable by any of the associated class decks. For instance, the Kabal Courier (three mana, two attack, two defend) has a battlecry that discovers a mage, priest or warlock card.
Each faction also a special leader card, such as Kazakus (four mana, three attack, three defend), a Legendary Kabal card. His battlecry is to create a custom spell if your deck has no duplicates. A custom spell! This is a brand new thing, and apparently there are more than 100 spells you can make.
Dedicated Hearthstone website Metabomb (also published by Gamer Network) has a Hearthstone: Mean Streets Gadgetzan guide, which covers custom spells, all the new cards, and more.
Mean Streets of Gadgetzan card packs, as with Whispers of the Old Gods and Grand Tournament expansion card packs, will be available to buy in Hearthstone with in-game gold or real money from Thursday. As with other expansions there's also a pre-purchase Gadgetzan deal where you can by 50 Hearthstone card packs for the price of 40 (Ł34.99). The card packs will be available to open when Gadgetzan launches.