Blizzard has detailed Hearthstone's first expansion, the Curse of Naxxramas, which will be a series of PVE battles against AI bosses that should net players up to 30 new cards.
The new expansion is structured so that players will fight through five dungeons (being referred to as 'wings') earning cards for defeating the bosses and completing specific class challenges. Each boss beaten will earn you a certain card while clearing a wing and all the bosses in it will earn you a legendary. Of the 30 cards there will be "five or six" legendaries according to game director Eric Dodds. The suspicion seems to be that there'll also be one extra boss or confrontation after all the wings are complete.
The bosses themselves will be new characters with Dodds summing up the structure thus: “Each wing is just a series of boss battles so you can expect to see each of the bosses that you would have seen in the Naxxramas Warcraft dungeon in order and you’ll battle them in order. It’s 15 bosses and then there are nine additional class challenges, and then there’s the few additional legendaries. So when you beat the final boss you get a card and then a legendary for clearing that wing.”
The new bosses will be built around particular mechanics and powers, the example given being Heigan the Unclean's ability to spend one mana point inflicting two damage to the minion on the far left hand side of the board. The emphasis it seems is on a puzzle element and building a deck to overcome particular abilities. Class challenges on the other hand will focus on defeating bosses with pre-built decks.
Currently each wing is set to be released weekly with the first one free. There's no pricing yet confirmed for the others.