Protoss:
Mothership Core - Build at Nexus and remains attached to it. Can warp to any other Nexus you control. The core is early game unit, can upgrade to full Mothership later. Core has mass-recall ability. 150 Energy. Ability: Purify: Becomes a very strong static defensive unit. Ability: Energize. Fills target friendly unit's energy to maximum. Mothership loses mass cloak.
Mothership - Upgrade from core requires Fleet Beacon. Loses mass cloak field. Vortex now affects ground units only. Ability: Stasis. Locks mothership and friendly nearby units into protective stasis for 20 seconds.
Tempest - New air siege unit. Attacks air/ground. Upgraded range: 22.
Oracle - Cloaking ability like mothership's old cloak ability. Mass cloak costs energy. Ability: Preordain. Cast on any building to turn it into detector for you. Mass cloak only lasts a certain amount of time. Ability: Entomb. Encapsulates minerals in an energy shield, making it impossible to harvest from them. Can destroy these shields.
Terran:
Battle Hellion - Transform a Hellion into a walking mech unit. Becomes a durable close-range combat unit. Meant to withstand late game chargelots, but still get torn apart by banelings.
Widow Mines - New spider-mine type unit. Costs supply, built directly at Factory (can be reactored). Can move around the map, when activated they latch onto the nearest ground/air unit and explode after timer (10 seconds). Opponent can kill own unit with widow mine on it to counter AoE. If a widow mine latches onto a ground unit and explodes, splash only hits other ground units. Ditto for air units.
Warhound - 2 weapons: cannon with bonus damage vs. mech and missiles that autocast on closest mechanical target.
Reaper - Now regens out of combat. Loses bombs vs. buildings.
Battlecruiser - Ability: Redline Reactor. Temporary speed boost.
Zerg:
Swarm Host - Slow moving ground unit. Primary attack is used while burrowed, spawns a few small Locusts to attack air + ground targets. Locusts will chase for quite some distance until they die. Recast on locusts is 45 seconds.
Viper - Flying Unit. Ability: Blinding cloud. Reduces enemy bio units range to 1. Ability: Abduct. Yanks enemy units towards them. Ability: Consume. Devour friendly building life to regen energy.
Ultralisk - Ability: Burrow Charge. What it sounds like. Bursts back out of the ground and it looks like it does damage when it erupts. Loses Frenzy passive and can now be snared by Fungal, etc.
Hydra - Speed upgrade at hive tech. Upgrade lets Hydras move as fast off creep as on creep.
Overseer - throws changelings. Changelings stick around longer now.
All Races:
Active Worker Count - Nexus/Hatchery/Command Center now have a counter showing the number of harvesters working nearby resources to make it easier to maintain optimum saturation.
Map Features:
Collapsible Rocks - New obstacle that partially blocks access ramps/lanes. Deal 500 damage = becomes wider Destructible rocks.
Other New Features:
New SP Campaign - 20 new missions, Kerrigan ever-present as a hero unit. Upgrade paths from WoL are more or less still in, but you're now upgrading units and Kerrigan's powers.
Custom Model Import - Blizzard has promised mapmakers the ability to import their own 3D models into their creations.
Global Play - Can match against players across regional boundaries.
Custom Cinematics - Mapmakers can script their own cutscenes.
Arcade - Improved custom map searching & browsing.