Terrans have some new toys too, but since Wings of Liberty was all about them, the additions are less dramatic. The Battle Hellion is a mutation of the Hellion, and can transform into a walking bipedal mech. Their napalm range/speed has been reduced; instead, they get a boost to their armour, and a wider spread of flames. They'll be good for controlling choke points.
The Thor has been rejigged too. Now you can only have one on the battlefield at once, and they cost twice as much to build, but their damage has been massively increased. The Warhound, on the other hand, is a cheaper alternative; easier to produce, and equipped with a powerful anti-armour attack that can chew through heavier units.
Finally, the Terrans have the Shredder. This robot is useless while in transit, but can bury itself in the ground and unleash a wide area of effect radiation field, taking out smaller enemies like Zerglings with ease. The catch is, it'll shut down if any friendly units are near it. You have to keep it away from your main army.
The Protoss have lost their carriers and motherships, but have gained three new units with some pretty dramatic, and unique, battlefield advantages. First up is the Tempest, which is a direct response to the Zerg's Mutalisks. They have anti-air area of effect attacks, as well as a ground attack.
The Oracle is a strange new unit with three powers. With Entomb, it can temporarily prevent a mineral resource from being harvested, slowing down the opposing team's means of production. Preordain allows you to scan enemy structures and find out exactly what tech is being researched, and what units are being produced. Phase Shift will 'stun' a structure, stopping anything it's researching or producing. We can see the Oracle causing some drama in multiplayer.
Lastly, the Protoss have a unit called the Replicant. Interestingly, it can actually replicate some enemy units. This evens the odds during battles, allowing you to create Protoss Siege Tanks, or Ghosts. The only problem with this bizarre new unit is that it costs a fortune to produce, for obvious reasons.
It's an interesting bunch, and will rewrite many of multiplayer's established rules and tactics. But even if you don't care about playing online, there's still a whole new story to enjoy, which sees Kerrigan building a Zerg empire. Characters from Brood War will make an appearance, and will make reference to the choices you made in the Wings of Liberty campaign.