Terrans
The new Terran units are more than meets the eye. Much like your friendly neighborhood Autobots, the new-and-improved Battle Helion upgrade allows the Hellion to transform from fast-moving flamethrower buggies to slower, tougher mechs that deal fiery splash damage attacks.
Hellions have been taking notes from Optimus Prime.
The Warhound, at first thought to be replacing the Thor, is basically a smaller version of that huge mech with a specialty in dealing mechanical damage (meaning it's not at its best against the Zerg). Its two attacks allow it to fire long-range missiles to break up sieges and a basic cannon for ground attacks. Unlike the Thor, it's completely defenseless against air attacks.
The best part of Widow Mines? They can jump high enough to attach to air units as well.
Terrans also get a new defensive (and area denial) unit known as Widow Mines. These little buggers burrow and stay out of sight until an enemy passes by. Then they jump out of the ground, latch onto the enemy unit, and deal a significant amount of area-of-effect damage -- enough to destroy minor units in a single blast. The best part? They can jump high enough to attach to air units as well. Protoss
The Mothership Core is by far the biggest change to the Protoss. This bouncing baby Mothership has three abilities that make it a no-brainer for boosting your army's power: it can recharge the energy of any unit with Energize, defend your base as a turret with Purify, and transport a group of units out of harm's way with Mass Recall. When you're ready to go on the offensive, it can be upgraded into a full Mothership.
The Tempest look a lot like the Dralthi from Wing Commander.
The Oracle is a pest. Not only can it entomb mineral deposits to slow down the enemy's economy, it can also cast a cloak field over friendly units to make them invisible and impossible to target without detection, like a mini Mothership.
If you're still mourning the loss of the Carrier, cheer up -- you might be happy with its replacement. The Tempest might be the most expensive air unit for the Protoss other than the Mothership, but its tremendous range is worth every crystal. However, its damage output is fairly low, so don't expect it to work as a powerful siege unit as much as in a support role, one that helps to scatter enemy units that are huddled together (like Siege Tanks).
Zerg
It may not look like a lot has changed for the Zerg, since they're only getting two new units, but there's a lot more going on under their gooey biological hides. For example, Ultralisks now have a burrow-charge attack that allows them to quickly close the distance to a target, and Hydralisks have an upgrade that lets them move faster off of the creep. Those are incredibly important changes that will make those formerly slow and lumbering units better able to keep up with upgraded Zerglings, and more useful in lightning strikes.
But yeah, there are two new units: The Swarm Host finally provides the Zerg with a much-needed siege unit. It's defenseless above ground, but when burrowed (and thus invulnerable unless the enemy has detection) it constantly spawns a pair of short-lived Locusts every 20 seconds that attack enemy units and structures. This is as close as the Zerg will get to having their own version of the Terran Siege Tank, though it still can't attack targets in the rear -- that's still the late-game Brood Lord's job.
Most of the work that has gone into the Zerg has happened behind the scenes for existing units.
Then there's the Viper, the new Zerg flying caster unit. It has no direct attack, but it can use Abduct to physically snag and pull both friendly and enemy units closer, and it can cast Blinding Cloud that creates a green cloud over enemy units lowering their firing range. It can also cast Consume on a friendly structure; the Viper to regenerate its casting energy, but at the cost of the structure's health. I could see building a forward Hive just to serve as a fueling station for these nasty things, and then using that energy to pull in ranged units for the kill. Maps and Tweaks
So far it looks like the biggest change in the way maps work will be the collapsible rock piles. Instead of simply having areas of the map that are already blocked off like in Wings of Liberty, HotS will allow you to destroy a rock tower that will scatter its crumbled rocks to block an area off. So players can now go ahead and create their own chokepoints, or buy themselves some time to get an expansion on its feet before the enemy ground forces can reach it.
Sure hope you marines meant to do that.
There are some smaller additions as well, like showing the hotkey buttons over corresponding units, or giving you a running count on how many worker units are currently assigned to mine a given deposit of minerals or vespene gas. You can also elect to play in ranked or unranked matches now.
These are just a few of the additions and changes to HotS that are currently in place today, and tons and tons will almost certainly change by the time it launches. If we learned anything while playing the Wings of Liberty beta a few years ago, the world of StarCraft II multiplayer is an ever-evolving beast.
I've noticed over all my years of playing StarCraft that it's a lot like a relationship. Both of us need to be willing to adapt to our ever-changing needs, and that every couple of years we need to shake things up to keep our relationship feeling fresh. Now that I think about it... is Kerrigan single?