I'm going to go ahead and update this thread with the rest of the changes that have happened since the thread got buried. This will be a long update, you've been warned!
Terran
Infantry:
Marines - The all purpose unit hasn't changed since it's initial showing. Still gets stim packs, health and range upgrades.
Marauders - This new unit is the Marines best friend. A bigger, stronger, slower infantry unit that shoots grenades. The grenades slow down all organic units (Zerglings, Zealots, Marines, etc) and does bonus damage to heavy units (Tanks, Immortals, Ultralisks). Not great by themselves, but perfect when used as a support unit to Marines.
Ghosts - Still unchanged. Can cloak (turn invisible), drop Nukes, Snipe targets, shoot EMP missles, and detect units with energy in the fog of war. Don't leave home without them.
Reaper - The jet pack wearing prisoners haven't changed much outside of getting bonus damage vs. small units. This extends their usefulness through out the end game.
Metal:
Hellion - Take the old Vulture and merge it with the Firebat and you get the Hellion. It's basically a fast moving 4 wheeler that shoots long streams of splash damaging fire. The Hellion is great for killing packed groups of enemies or raiding workers.
Viking - This unit can be upgraded to make it transform from a ground to ground attacking unit into a air to air attacking unit. The flexibility of this unit makes it like a Marine, not great at any one task, but not bad at anything either. All around useful.
Siege Tank - Nothing changed and honestly didn't need to. Great unit.
Thor - The huge lumbering Thor has been tweaked a lot since it was first shown and will probably continue to change before the game releases. It's primary purpose is against air units as it does extra damage to heavy air units. It also has a special ability to repair itself after it has been destroyed in the field. If the enemy doesn't finish off the wreckage, it will begin to repair itself.
Air Force:
Banshee - An air to ground bomber that can cloak. A definite must have support unit to any ground force.
Nighthawk - The new name for the Nomad. Basically a flying spell caster that can deploy spider mines, auto-turrets, and targeting drones. It's also the only unit that can detect invisible and burrowed units for the Terran.
Battlecruiser - The juggernaut of the Terran air force. High hit points, powerful weaponry, but slow and expensive. Can be equipped with either a single shot high damaging Yamamoto cannon or with a anti air cluster missle barrage. Choices...
Medivac Dropship - The Medic has now been fused with the dropship to create the Medivac Dropship. This unit can carry up to 8 supply worth of units in addition it will be able to heal infantry units within range. Still being tweaked to make it viable.
Zerg
Basic mutations:
Zerglings - Fast, cheap, and low hit points. Attack early and in swarms to drive your opponent nuts.
Banelings - A Zergling mutation. Banelings are rolling bombs that deal splash damage to any unit it runs in to. Devastating vs infantry and buildings.
Roach - A ground to ground ranged attacker that can quickly regenerate it's hit points. In order to kill them, a player must focus fire on them. Use them to absorb enemy fire so that Zerglings and Banelings can get close for the kill.
Advanced mutations:
Hydralisk - The Hydra has gotten better at killing air units and has gotten stronger all around. A staple of any balanced Zerg force.
Lurker - An evolved version of the Hydra. The Lurker loses it's ability to shoot air, but can attack while burrowed. In addition, it's attack hits as splash which allows it to shred groups of enemy infantry.
Mutalisk - The fast, weak, harasser of the Zerg air force returns unchanged. It's still really good at what it does, killing workers and stray infantry.
Corruptor - A air to air unit that shoots spores at it's enemies. Once the enemies hit points have been depleted, they become infested and will start shooting on their own units for a set period of time.
Super mutations:
Ultralisk - The mammoth Zerg ground unit. It can attack 4 units at a time with it's claws and has a huge amount of hit points. Sending these in with Zerglings and spell casters usually means the end for any opponent.
Swarm Guardian - A long range air to ground bomber that infects it's enemies with it's attack. Every hit on an enemy spawns little critters that will attack and harass enemies until they are killed or run out of energy.
Infestor - The Zerg spell caster that can move while burrowed. In addition to casting a variety of crippling spells, it can infest buildings. Once infected, the buildings will begin spouting out Infested Marines for a short while. Instant army!
Special mutations
Overseer - An upgraded Overlord. The Overseer can detect units, has increased vision while not moving, can heal Zerg units nearby, morph itself into a Nydus Worm to transport Zerg armies all over the map quickly, and create Changelings. Changelings are short lived spy units that assume the form of enemy units to allow you to sneak into a enemy base undetected.
Queen - The hive mother of the Zerg. She is primarily used as base defense and has spells that will heal or upgrade buildings around her. As you go up the tech tree, the Queen will mutate as well, gaining more hit points, magic, and spells. You can only have one Queen at a time.
Protoss
Ground units:
Zealot - Unchanged from it's original form. Hard hitting and harder to kill ground to ground attacker. Can be upgraded to have a "dash" move that helps them close in on their opponents.
Stalker - A unit that can teleport over short distances or up/down cliffs. Very useful for harassing workers or killing off retreating units.
Immortal - A powerful unit that has a special shield that reduces all heavy attacks against it to minimal damage. Great at destroying Siege Tanks or Ultralisks, but relatively helpless against smaller units like Marines and Zerglings. Can take a Nuclear Missle blast and walk away with only 10 damage done!
Nullifier - An early game support unit to the Protoss ground forces. Uses a force beam on opponents for little damage, but slows down their movement and attack speeds. Can cast spells such as Anti-Gravity (picks up a unit or building and holds them in the air) or Force Field (creates an barrier that units have to move around).
High Templar - Another spell caster. This unit can cast Hallucination and Psionic Storm. In addition, two Templars can merge themselves together and form a Archon.
Archon - High damage, high shields, splash damage, hits air and ground. What more do you need?
Dark Templar - Permanently cloaked units that do large amounts of damage to any ground units they encounter. One of the weakest ground units for the Protoss, so keep them away from detection units and buildings.
Collossus - A massive unit that can walk up and down cliffs easily. Hits enemy ground units with a splash attack laser. Can also be upgraded to have a larger shield to absorb more damage. The drawback? It can be hit by both air and ground units because of it's size.
Air units:
Pheonix - An air to air specialist that can overload it's systems to do massive damage to any units nearby. Afterwards, the ship goes offline and is helpless.
Phase Prism - One part transport, one part energy provider. Having a Phase Prism deploy will allow the Protoss access to any spells or buildings that require Pylon power. Use it in conjunction with Warp Gates for fast reinforcing of the front lines.
Void Ray - A flying unit that uses a focused laser beam that does increased damage the longer it's deployed. Great against strong units with high hit points and buildings. Vunerable to smaller units like Vikings, Phoenixs, and Mutalisks.
Carriers - The flying blimps return with little change. They deploy smaller swarming fighers to attack from a long range.
Observers - Small cloaked satellites that can detect other cloaked units. Great for spying all over the map for little risk.
Mothership - The biggest spell caster in the game. She can cast Time Bomb which nullifies all attacks against it for a short duration. She can also use her energy to recharge other spell casters around her. Finally, she can teleport units to her position on the map.