Wings of Liberty wasn't that hard either, but I do remember Supernova giving me a lot of trouble when I first got to it, and still being pretty rough on subsequent playthroughs. The only mission that really gave me trouble in Heart of the Swarm was Fire in the Sky, and it's not so much that the mission is hard, it's that it expects me to do the whole thing off one base Zerglings and Banelings. I like the composition, but it isn't a cost-effective one.

Incidentally, that ties in to my major complaint about the game. It didn't feel like I was playing Zerg at all for most missions. Zerg air is undervalued, Overlord drops and Nydus Worms aren't in the game, and most missions force you to play off at best two bases. A more spread out playstyle would have increased both the Zerg feel and the difficulty, I believe.

It was still fun enough, and multiplayer is looking really good.