I hate the focus on Kerrigan as a hero unit... complete with her own upgrade path. Ugh. Really didn't serve the gameplay at all. Sure, WoL had some heroes, and the obligatory hero only units... but you'd actually manage your army. All you have to do in this game is press F2, attack move, and spam abilities. I never really even felt the need to build base defense or anything. A far cry from the bunkers and heroic SCVs who fixed them in WoL. Who needs defense when you can conjure up six banelings out of thin air and oh when they explode they can also heal you...
Not to mention being able to summon a Zerg Death Star every 5 minutes. Good God, is she unbalanced! It would have been an entirely different game without her.