Most people are upset that the original Kerrigan, Glynnis Talken, is not returning as the character, and there have been many rumors about who Blizzard was gonna bring on.
I had heard that Glynnis Talken's voice had changed and just didn't sound right for what they wanted for Kerrigan. I know it sounds kinda cheap but it happens.
They did a sample of her at the panel, but it was crappy audio over the stream so I couldn't make it out. From what I gather she's a more recent addition to the cast, and the older trailers likely don't have her.