I just beat the Heart of the Swarm campaign, and I have to come right out and say I absolutely loved the story. The gameplay was really fun (more on that in a moment), but what kept me pushing through all of the missions was an unquenchable need to know how the story progresses. I wont to spoil anything, but I felt a lot more connected to the characters this time around. The story kept me engaged throughout, and began building up (not destroying as some people think) the Wings of Liberty campaign. Heart of the Swarm paved a road right to the next expansion.
Gameplay wise, HotS offers a lot of variety. Normally I don’t enjoy the ‘take control of a single character’ type of missions, but in the HotS campaign I felt Blizzard added a lot to make it feel almost… almost closer to an RPG (dare I say WoW?). The boss battles were somewhat hokey, and definitely contrived from the WoW boss experience: Don’t stand in the fire. But they’re fun, and add yet another layer of non-standard gameplay. Hero abilities and mission objectives were diverse and I can’t say I ever felt the need to rush through a mission because I was bored of a particular mechanic. In fact, most of the missions ended rather quickly (~20-30 min tops).
Upgrades are handled a bit different this time around. For most of the zerg units you get to choose 1 of 3 specializations, but you can change them any time you’re not in a mission. For example, two of the choices for Zergligns are the classic speed increase and the attack speed increase; both are normally upgraded in-game, but in the campaign you choose between them pre-mission. You also get to do the same for Kerrigan by picking many different abilities for her to use as she levels up and gets stronger throughout the campaign. Lastly, there are mutations which act as permanent ugprades for the core units. These mutations are actual mini/short missions where you obtain the genetic alterations and get to test them out briefly before having to choose which of the two mutations you want applied. The mutations definitely impact strategy.
Overall, I loved it. I’m going to eventually replay on Brutal mode and see if I can get a few more of the portrait rewards. Definitely worth the price of the expansion without even taking multiplayer into consideration.