Terran:
The Command Center can immediately be upgraded to either a Surveillance Station or to a Planetary Fortress. This upgrade requires a lot of Vespene Gas, however, so it’s not actually available right at the beginning of the game.
Aside from the Supply Depot and Vespene Gas Refinery, the Command Center also gives access to the Engineering Bay, which allows SCVs to build Missiles Turrets, and Sensor Domes which can be upgraded to Radar Domes.
The Terran are unique in that the three unit producing building can have add-ons attached to them - either the Tech Lab, which unlocks more advanced units, or the Reactor, which effectively doubles the building’s unit producing rate.
The first unit-producing building available is the Barracks, which trains Marines, Medics and Ghosts, though only the Marines are immediately available. The Tech Lab unlocks the Medics.The Bunker is now unlocked and can be built by SCVs.
The Barracks opens two paths for the players to choose from:
1) Merc Haven. This is where the player recruits Reapers. These are not trained normally - the building slowly “generates” Reapers, which can then be quickly recruited (for the right cost).
2) Factory. This is where the Terran build their armored, mechanized units: The Siege Tank, Viking and Cobra. Only the Viking is immediately available - the Siege Tank requires a Tech Lab.
The Factory opens up two more paths:
1) Armory. This building unlocks the Cobra, and allows construction of the Munitions Depot, which provides the SCVs with the option of constructing a Thor.
2) Starport. Predator and Dropship are immediately available, while the Nomad and Banshee require a Tech Lab add-on. Battlecrusiers can be built once the Deep Space Relay building is constructed.
The Starport itself can be upgraded to a Star Base, which can lift off and still produce units. However, since add-ons stay on the ground, it is not clear at this point whether the Star Base will be able to produce Banshees and Nomads when airborne.
The new Terran Tech Tree allows the Terran player more options in his build than in StarCraft 1. Ghosts no longer require a full tech tree, with a Starport, Science Facility and an add on. The Thor, the supposed Terran “super unit”, can be acquired relatively early on, especially since the Armory will likely have necessary upgrades for other Terran units. Only the Battlecruiser, the Terran capital ship, requires almost every piece of the tech tree to be built.
It seems that Blizzard is attempting to make every multiplayer match of StarCraft even more diverse and unique than it was before, with more options to choose from and with more strategies to try, instead of having to follow a linear path of technology.