Blizzard compensates for this with "Evolution" missions and RPG elements that give you a great deal of control over how Kerrigan and the Zerg themselves evolve. Kerrigan's abilities are customizable and expand as she gains levels and progresses through the game. Optional side missions offer the ability to gain additional levels and thereby increase her power and damage.
In between missions, you can explore the Zerg space-going Leviathan or speak to the Evolution Master (Abathur), your chief advisor (Izsha) or some of the other characters who turn up along the way. According to Blizzard, Rage, fury, and revenge are the major themes of HotS, but that's not what drives the game. The big question of Starcraft II: Heart of the Swarm is whether or not Kerrigan can retain any part of her humanity while simultaneously reunifying the Zerg and attempting to destroy the Terran Dominion. Blizzard does an excellent job of guiding Kerrigan's evolution (pun intended) and sets up the third game, Legacy of the Void, deftly.
Single-Player GameplaySingle-player gameplay is pretty great. Kerrigan is a regular battlefield presence and can respawn if killed in most missions, freeing you up to use her in an offensive capacity. As with Wings of Liberty, Blizzard splits gameplay between single-player and multiplayer; there are a number of units available in the campaign missions that can't be used for multiplayer.
This is where the Evolution missions come in. As you play through the campaign, Abathur will periodically offer Kerrigan the chance to upgrade a Zerg unit into one of two distinct types. Which type you choose has an impact on how you use that particular unit afterwards; Roaches can evolve into Corpsers (Move while burrowed) or Viles (debilitating attack slows enemy movement and attack rate). That's in addition to the evolution options you can pick for each species on a mission-by-mission basis.
The problem with the Evolution missions is that they aren't much fun. They're designed to give you a look at the particular capabilities of each of the two optional Zerg strains, and while they do that well enough, it's boring to walk through a pair of missions with extremely simple goals -- particularly if you've already chosen which of the two strains you prefer to use. It's a great idea that would've worked much better if the evaluation had been baked into other side-quest missions or optional plot explorations.
Missions and Plot LinesThe strength of Starcraft was always the way the three races felt different, and the single-player campaign of HotS retains that characteristic. The Zerg remain a ferocious, organic, rolling ball of danger, but the new evolution options let you customize the Swarm and Kerrigan to your own liking. Missions are also broken into multiple different stages and types. Some missions alternate between hero control and base building, while others center around different Heroes or enemy infiltrations. The game encourages you to use Kerrigan as an offensive asset and there are multiple ways to fine-tune her abilities for faster base building or better personal firepower.
The battle against Mengsk and Terran Dominion forces is a bit too easy on normal difficulty, but the payoff and setup for the next game are satisfying. After Diablo III's disappointing story, Starcraft II proves that Blizzard is still capable of a decent popcorn movie plot line. The cut scenes, as always, are amazing.