Panel over, all the info and some thoughts below.
The new Hearthstone expansion, some details of which were already reported, has been revealed as Gnomes vs. Goblins in the Blizzcon 2014 opening ceremony. It contains 120 new cards, increasing the current pool by about 25%. The focus is on the explosive developments of the Warcraft universe's two diminuitive races, introducing Mechs as a creature type and with a focus on more random effects. Battle.net's got the details including some new card previews and the announcement trailer is below.
Coming with the first expansion, described as such since the Curse of Naxxramas was a single-player focused complete package that wasn't purchased through boosters, is spectator mode. There's some small detail on the announcement post, which mentions you'll be able to jump into friend's games if they have it enabled, but no shots of how it looks yet.
The new cards have a variety of new abilities, including "Mega-Windfury" which allows them to attack four times in a single turn. There's also a new game board, shown off in this screenshot. I'd expect there to be more reveals at the panel happening right now. We'll keep you updated.
Designer Eric Dodds introduced the expansion properly at the panel. He explained that the goblins were focused on direct damage and killing things, while gnomes will often turn other cards into different stuff. This is showcased by two minions: a goblin who comes into play and deals damage to a random enemy and a gnome that turns another of your minions into a random one of the same cost.
He then moved onto mechs, saying they wanted them to become a viable choice as something to build a deck around. Different classes will have their own mech cards and the modifier will also be applied to some older cards. While there will be a number of regular minions, it's a theme for them to have the ability where, upon death, they'll spawn another minion. As Eric pointed out, this is a very powerful effect as Hearthstone games often revolve around removing all minions from an opponents board as often as possible. The three cards shown as examples of this escalated in power of produced minion as they increased in cost: a random 2-cost, a random 4-cost and then a random legendary.
Dodds then mentioned that Goblins vs. Gnomes will be distributed through special packs that are earned or bought in the same way as normal packs. They'll also be craftable with the same arcane dust that base set cards are made with and disenchant into.
Producer Yong Woo came to the stage to chat about other changes coming to the game, such as spectator mode. It attaches to one player's perspective, so can't be used to help friends out with some advance knowledge of what's happening in an opponent's hand. You'll be able to see what that player is doing and what cards they're focusing on. For the professional scene you'll be able to spectate both players simultaneously and stream everything through one client. This will be welcome news to the long-struggling tournament organisers who have so far been jerry-rigging it via recording players screens and should allow for online tournaments with casting.
He finished by saying the expansion will be out next month, before the end of the year.
Adding a lot of random effects to the game is an interesting choice. Game Designer Ben Brode actually came onto the stage earlier on to talk about randomness is games and how skill and luck interact. It was an interesting talk, pointing out that something like chess often comes down to rote memorisation rather than reactionary skill, while Noughts and Crosses has no luck but no skill either. However, while those things are true, it won't stop the immediate reaction to every poor coin flip being enraging for players and potentially drive them away from the game. It's also a balance worry - you don't want these cards with random effects to show up too much at times when that randomness can't be manipulated. It will be interesting to see how the Hearthstone community, particularly the pro scene, reacts come December.