I also scratched my head at the Matriarch's corruption. But I think this puzzlement is actually intended as Kerrigan's strength is severely underestimated by the Protoss. And by having the player wonder about this issue, he is forced to rethink his assessment of Kerrigan and the threat she represents, while he is also shown how the pride of the Protoss clouds their judgment. Something which I feel didn't really come across in the main game's Protoss campaign.

That said, while I was a bit disappointed by most of WoL's storyline, I am pretty hyped up for the next SC2 game.