From the few hours I've already put in competitive SC2 HotS, I can safely say that the whole multiplayer is changed due to those new units. Not having detection early game is severely punished now with the Terran Widowmines and cheaper DTs. I haven't seen much of the new Zerg units being used multiplayer yet, though the speed upgrade for hydras make them a more common sight on the scene now compared to WoL.
My favourite change so far is the lack of Siege Tank Upgrade. All Siege Tanks start with Siege Mode pre-upgraded, allowing Terran to get a powerful defense against Tier 1 spam quickly.
My favourite change so far is the lack of Siege Tank Upgrade. All Siege Tanks start with Siege Mode pre-upgraded, allowing Terran to get a powerful defense against Tier 1 spam quickly.