Yeah, the motivation for the love story was weak. My understanding is that some of the novels shore it up a bit, but obviously that doesn't do much for the game. I still don't mind Kerrigan's response, though. Even if they haven't had time to develop some deep emotional connection, Raynor is still all Kerrigan has, and she's is in a very vulnerable emotional state. To say nothing of the fact that she's not exactly a balanced individual in the first place. I can definitely see an amnesiac, vulnerable, angry (and more than a little bit crazy) type-A personality deciding to enact vengeance on those who took Raynor away.
What was wrong with that? Firstly, Isha (or however you spell that) apparently retained all of Kerrigan's memories. Maybe she brought Kerrigan up to speed (psychic-ly or otherwise) on the Zeratul situation. Even if you assume otherwise, though, you have an unknown dark templar uncloaking in the heart of Kerrigan's stronghold. What is she supposed to think? An immediate, violent response seems pretty understandable to me.
Yeah, and this was mostly what I was referring to when I talked about the "story." The plot isn't amazing, but the technology, the pacing, etc, was excellent. I felt HotS was one of the few titles I've played that struck a solid balance between "movie" and "game." There are a lot of games that you watch more than play, and a lot of games where you've totally forgotten what was going on by the time the next cutscene plays. HotS, though, got it right.