Drag Queen - taking a hint from Namco, Blizzard decided that a little lipstick and a bow is all you need to reinvigorate a character from a previous game. Ensnare no longer uses green slime, but FABulous multicolored feather boas.
Overoverlords - Zerg now has a second layer of control. You only need the one to make enough overlords for 200 control, but a replacement costs 2400 minerals if your opponent destroys it. This is Zerg's unique super-unit. At hive, you can upgrade it with creep-lift capability, allowing it to wrap up entire Zerg bases in a tidy bundle and carry them to new locations.
Metalisks - replacing the ultralisk, the metaalisk is now a transport type unit which carries zerglings into battle, protecting them until they can be released to attack in swarms, like interceptors from a carrier. Features jet pack.
Nydus Worm - replacing the Nydus Canal, these no longer need to be targetted at a particular location covered by creep, but instead, once built, they burrow their way to a completely random location without creep.
Cereblings - a cross breed between the cerebrate and the zergling. These free your hands by taking command of 12 units (of their choosing), and directing them with the full competence of the computer AI. You may no longer give orders to units once they have impressed on a cerebling. Cereblings report directly to the Overmind or Kerrigan and do not take your orders.
Degrader - replacement of the defiler. Releases pheremone sprays that rob medics of their competence, turning them into ineffectual bimbos who go dance for tips at the Merc Haven (however, this increases reaper recruitment rates).
Zerglings - not new, but upgraded! Now zerglings look exactly like grasshoppers, and make cute little boing boing sounds as they hop along. Zerglings no longer do damage themselves, but can be evolved into banelings or cereblings.
Banelings - testing revealed that mobile banelings were overpowered. We have balanced the baneling by keeping it in its immobile egg form permanently.
Infested Terrans - for the sake of consistency, all infested terrans now form a chrysalis and evolve within for 3 hours before emerging and exiting the play map to challenge Kerrigan for dominance of the Zerg.
Pyrolisk - substitute to the Hydralisk. Answer to the Firebat. Their flame-throwing attack is essentially the same as the Firebat's, except that they do friendly fire damage, as does the explosion they make when they die. Range and speed upgrades no longer available.
Volcalisk - evolved form of the pyrolisk, replacement for the lurker. Burrows permanently (this takes some time, as the unit must cut a channel deep into the earth), then sprays lava in random directions.
Fireeater - alternative evolved pyrolisk form. Has no attack, but once burrowed, eats reaver scarabs and sprouts minerals. Excellent for base defense!
Whisk - replacement of the scourge. An answer to Terran's irradiate ability (not featured in SC2), these suicide support units crash into friendly units and cure their status conditions.
Glaive Wurm - glaive wurms have now achieved hive mind sentience and independence from the swarm. They consume mutalisks shortly after morphing to create a wurm boil. Like the reaper, they can be hired as mercenaries, and loaded onto overlords or metalisks to attack.
Defendor - replacement to the guardian. We at Blizzard are offended by the aggressive uses of the guardian, which was intended to be a defensive unit. Defendors now can only morph directly from larvae, and remain immobile (though flying) once morphed.
Devourer - unchanged.
