Campaign? Check. Multiplayer? Check. Balance? Well...
The campaign in HotS is simply amazing. Blizzard worked very hard to make this game feel less like a series of missions and more like a flowing narrative. The acting, character animation, story and so forth are all very well put together. Upgrading, or "evolving" your units could have easily felt directly lifted from the upgrading you do in WoL, but even there Blizzard found ways to improve the experience, giving you some permanent choices, peppered with non-permanent, loadout style upgrades.
HotS seems to pull gameplay inspiration from just about everywhere, at times the game goes full on MOBA, and those moments make for some of the most fun I've had in an RTS, ever. For much of the game, you have The Queen of blades under your control. She plays like any other hero unit has in Blizzard games, but is by far my favorite. Controlling Kerrigan is a joy, and the ways in which she grows stronger throughout the game reward players who use her tactically. The Queen of Blades has her own leveling system, and she can be customized to make missions playout somewhat differently. You can set her up as a high dps ranged unit, you can charge through lines of enemies bashing them with melee, or you can even tank a good amount of damage with HP and healing upgrades.
The multiplayer needs a few weeks for things to be felt out, but that is fine by me because it gives me time to try and 100% one of the best campaigns in RTS history. HotS is easily worth your money, especially at the tasty price-point of only $40.