The Zerg Viper air unit can pull units closer to its forces.
For instance, Terran players are used to playing fast and hitting hard. They don't have a particularly strong defensive army which makes them vulnerable if caught outside their base without a positional advantage (e.g. high ground, choke point etc.) Their new units (particularly the Widow Mine) address that and give them the ability to dig in and hold control of a zone. Protoss players were weak in the early game, but the Mothership Core and Oracle give them much-needed opportunities for aggression. Zerg players didn't have many options for defeating an enemy besides ... well, running straight at them. Their new units, the Swarm Host and the Viper give them a chance to besiege their opponent and engage with more precision than before.
None of the additions or changes are particularly important individually. What matters is the cumulative effect they have on the flow of the game.
HotS is a much faster game than its predecessor, and aggression is more prevalent at all times. Whereas WoL tended to devolve into long, economically-focused games, HotS encourages faster play with more harassment of your opponent. In WoL, the best defense was a good economy. In HotS, the best defense is a good offense.
This is a godsend for players who were getting bored with WoL. The more rapid pace means that the skill ceiling has been raised as the game tests their multitasking skills. New and revamped units also mean it's more difficult to anticipate the enemy's moves.
Widow mines offers the Terrans a bit more defense.
Unfortunately, all of this is a double-edged sword. Newer players will likely find that the faster pace of gameplay means a steeper learning curve. Getting obliterated for your first 20 matches is already a fact of life (and a rite of passage) in SC2, but HotS could make that first hill even more difficult to climb.
To their credit, Blizzard has included a few new options designed to ease players into multiplayer. The principal challenge of StarCraft multiplayer has always been figuring out how to play StarCraft multiplayer. Unlike most games, the campaign teaches you very little about how the multiplayer game is played. Which makes the new tutorials and the addition of unranked matchmaking such a welcome inclusion. There are now multiple avenues for players to hone the skills of competitive play with each race before jumping into the shark pool that is ranked play.
No matter what sort of tutorials Blizzard includes SC2 will always have a steep learning curve. Thankfully, Blizzard provides players with the tools they need to study the game. As with WoL, all of your matches can be saved as replays, and you can review them to see where things went wrong or to view the game from your opponent's perspective.
Because these are actual game saves and not just video replays this allowed Blizzard the chance to include an extremely cool new feature called Take Command. You can now choose to watch a replay with a friend or previous opponent and take control of the game at any point. You can re-attempt a crucial engagement to see if better control might have saved your army, or you could retreat and try a new strategy all together.
The Protoss Mothership Core allows a mass recall of units, allowing early aggression.
A shadow has been looming over the release of Heart of the Swarm. The Zerg-focus of StarCraft 2's first expansion pack draws associations to the original game's first expansion, Brood War, which has often been called the greatest real-time strategy game ever made. It was Brood War that refined SC's multiplayer into a well-tuned and balanced force of competitive gaming, and it managed to support a thriving player base and competitive professional scene for over ten years until SC2 launched in 2010.
Is Heart of the Swarm the Brood War of StarCraft 2? It's going to take years before we have any idea if it will live up to that daunting legacy, but it's off to a good start. At the very least this provides a great boost to the community as we begin the long wait for the next expansion, the Protoss-themed Legacy of the Void. The final piece of the multiplayer puzzle, which will see StarCraft 2 achieve its full form.