As exciting as the campaign looks, it's the added units that are generating the most conversation.
Blizzard revealed a bit of information on 'Heart of the Swarm' this spring, but at BlizzCon we got full details - or as full as possible from an hour long panel.
The storyline for 'Heart of the Swarm' is what you'd expect based on the ending of 'Wings of Liberty.' You take control of Kerrigan in order to rebuild the Zerg over the course of 20 or so missions. There are also mini-missions during which you will choose different ways to evolve the species, giving your units new abilities along the way.
A plethora of new units have been added to the game, and not just for the Zerg. Terran players get an interesting new unit that does area-of-effect damage when stationary as well as the Warhound, which is like a smaller, faster Goliath.
Protoss get three new units starting with the Tempest, a very cool looking ship that does AOE damage to air units. The Oracle is an absolutely crazy ship that allows you to halt mining or eliminate certain technologies from other players by phasing out buildings, and if you're looking to really mix it up you can build the Replicant, which can turn itself into any non-massive unit. Want to mix a Siege Tank in with your army? You can make it happen.
The Zerg are getting a pair of new units as well. The flying addition is called the Viper and it has an absolutely crazy ability that pulls enemy units towards it - could be a lot of fun to drag something into a pack of Zerglings. The Swarm Host meanwhile is a bit Lurker like, but instead of simply attacking, it produces units when burrowed.
Source: PC Gamer