I won’t spoil the surprise, but Heart of the Swarm has not only the various unit upgrade features called Evolutions that Wings of Liberty have but also has a separate skill tree for Kerrigan herself as she levels up to unlock new abilities which can be either passive or active.
I bought 2 copies of the game, one for my son who is an avid StarCraft fan as well and one copy for myself and it was well worth it so far. I am not really a great player on multi-player, I tend to base build and have a weak offense taking my time so my skills are suited to custom battles against CPU or other casual players, certainly not the hardcore gamers online, and I won’t be entering any tournaments.
Here are some Heart of the Swarm Screenshots
StarCraft 2 Heart of the Swarm is definitely worth picking up if you bought Wings of Liberty and completed it, the new features, units and story are enjoyable and I missed playing. It was way too long between the release of Wings of Liberty and The Heart of the Swarm.