It is going to be AWESOME!...
I'll start off with the Protossunits.
The first unit Blizzard released in their trailer is the shillouette of the airunit which looks like a phoenix.
On the first look the unit is only able to splash in the air. I can't see another reason for this unit to exist then no reason LoL. The unit is in my opinion totally senseless cause they only defend mutalisks in a high number. They'll maybe give this unit another ability besides splashing so the unit has more reasoning to be implemented.
The next Unit blizzard presented in their trailer looks like some sort of a Sentry but the usage is a little different. The 'Replicant' copies a selected non-massive unit of the enemy, probably for a certain amount of time 'cause the Unit would be deadly OP if the mindcontroll would exist forever. Furthermore this units are able to denie mining through blocking the mineralpadges with some sort of a forcefield. Another important point for the Protoss is the new Nexus. The Nexus spawns with more energy then before and will grant the Protossplayer a new spell which casts a shield on a building aswell as somesort of an defensive cannon to help Protoss defend Drops easily.
Next up are the Terrans...
Terrans will not have such big changes as the Protoss have rechieved.
"The Battlemech". The battlemech is a transformed version of the hellion. It will be granted additional HP, more DPS but also a decrease in the movementspead aswell as the spread. The Battlehellion will work as a new "Firebat".
The "Warhound" is a Unit to help Terran defend the fearsome Protossdeathball. It will be able to deal aditional damage against mech. Units and deal AOE (area of effect) damage on Airunits. The unit looks and behaves like a mixture of a Goliath, a Marauder and a Thor.
Besides the 2 new Units the Terranrace will be able to research a speedboost for the Battlecruiser which is supposed to help the Terran to become a better positioning in the very Lategame.
Finally the Zergs and I have to say those new Units look deadly OP.
The.... Lurker?!.. :)
This is one of the new Zerg-Units in Heart of the swarm. The Unit will simply work like a SC1 Lurker. You will be able to maintain a possition on the Map while burrowed. The Unit shoots out "broodlings" which will attack any buildings and Units that are near the .... Lurker... I think it will only be able to attack while burrowed. The unit will be used as ther perfect Siegebreaker agains a Terran opponten whos has no to waste.
The next Unit that was released is the by far funniest unit I've ever seen. It's a new Zerg Air-Unit which allows the Zerg Player to grap any enemy unit and pull them towards the Zerg (or away depends where the unit is standing). It will be so much fun to simply pull those Tanks up on the highground to rape them appart :). In connection with Neural parasite you'll be able to get all the hightech dmgdealing units out of your opponents army to use them yourself and possibly kill them.
Besides those two AWESOME - srsly - GOD DAMN F*CKING AWESOME FUN units Zerg will be able to research the funniest sh*t I've ever seen. The Ultralisk will now be able to charge towards the enemies units while beeing burrowed. Omg, that's really f*cking OP! :)
Can't wait for HOTS guys, can't wait :)