A fascinating expansion, Heart of the Swarm takes everything that was good in WoL and makes it even better.
The UI of the game got a huge facelift. The multiplayer got a slight overhaul with addition of new units and abilities to each race, but it's mostly in the surrounding areas, where the changes really make the greatest impact. The leveling system, while imperfect, makes sure that losing a game is not time wasted, and the unlockable dances/portraits and unit skins are now only tied to this system, making the simple process of playing more rewarding.
Overall, it's a perfect expansion. If you didn't like the original, then there's nothing for you in this installment, but that's the point. SC2 is an RTS where change in underlying mechanics is impossible (how would you change chess?) but the polish and error-correction that Blizzard undertook really makes the game feel more cohesive and enjoyable in all aspects.