Alright, I just finished up the campaign, and I have to say that I found the storyline to ultimately be really disappointing in what was otherwise a really great game.
Given that we already knew about the hybrids from StarCraft 1 and we already knew about the Voice in the Darkness from the graphic novel, the only relevant changes to the StarCraft universe in Wings of Liberty were the broadcast about Mengsk and Kerrigan becoming mostly human. Since Mengsk is still in power and wasn't really that popular anyway (as witnessed in the beginning of the game), this makes Kerrigan the only relevant change.
Based on what happened in Wings of Liberty and what we know of the sequels, it seems that Blizzard is most likely taking one of two possible scenarios for the trilogy.
Possibility #1.) The Completely Irrelevant Plot Loop
In this scenario, we find out that the entire StarCraft II trilogy actually resulted in absolutely nothing happening to the universe. While Kerrigan was turned into a human, in Hearts of the Swarm she not only turns into a zerg again, but also returns to being evil. Arcturus Mengsk is killed at one point in the story, and his son takes over the Dominion. However, despite seeming benevolent at first, Valerian ultimately turns out to be just as evil as his father. Finally, in Legacy of the Void, the protoss manage to defeat the Voice in the Darkness thanks to Kerrigan's efforts against the hybrids in Heart of the Swarm, removing the hybrids and the Voice in the Darkness from the universe.
Due to this course of events, we find ourselves at exactly the same place we started, allowing StarCraft III to start with a drunk Raynor trying to start a revolution against (Valerian) Mengsk and moping around that he lost Kerrigan (again).
Possibility #2.) The Minor Plot Development Trilogy
In this scenario, the universe of StarCraft does actually move forward, but only a little. In Heart of the Swarm, Kerrigan once again becomes the Queen of Blades, but retains her humanity, fulfilling the Overmind's plan of freeing the Zerg from the corrupting influencing the Voice in the Darkness, much like Grom Hellscream sacrificed himself to free the orcs from the corrupting influence of the Burning Legion. Despite Kerrigan becoming a zerg again, she and Raynor continue their relationship, because Blizzard showed that they were perfectly fine with interspecies relationships with Thrall and Jaina. In Legacy of the Void, the protoss, with Kerrigan's and Raynor's help, defeat the Voice in the Darkness and his hybrid armies, though some pockets of hybrids remain, controlling relatively small amounts of zerg. Finally, at some point in the trilogy, Arcturus Mengsk is killed and the Dominion falls apart, but Valerian, seeking vengeance against his killers, takes a small group of loyalists and leaves to plot his revenge.
Due to this course of events, we find that the StarCraft universe has actually advanced, but just not very much. We also conveniently now have good/bad humans, good/bad protoss, and good/bad zerg, allowing for easy integration of (insert faction) vs. (insert faction) gameplay in StarCraft III without having to randomly invent a faction like the Tal'darim.
Relative to the original StarCraft, I don't find either of these options especially compelling, though it's possible the second option could at least be somewhat interesting with some really high quality writing. However, given what I feel is a steady, steep decline in writing quality from Blizzard ever since StarCraft 1, I don't hold out much hope.
I also have a very hard time imagining any other plotline given Blizzard's history, though I would love to be proven wrong.
If I had to guess as to which of these options they would go with, my guess would be the second, since much in the way they recycled StarCraft plot elements like Kerrigan into WarCraft (Sylvanas), I expect them to do reverse since they lost the Warcraft universe as a universe they could easily make major changes to without upsetting the basis of the gameplay. Since Blizzard was exploring the concept of redemption with Sylvanas before they realized that they couldn't change the factions in World of Warcraft, I expect them to move that storyline back to Kerrigan and leave Sylvanas as the eternally vengeful queen of evil, since that works excellent with the Warcraft faction split.
I guess if I ultimately had to explain why I felt so disappointed with StarCraft II's plot, it would be that I viewed StarCraft 1 as having really interesting plot with a few rough edges, and held out hope that despite the steady decline in Blizzard's writing quality, they would fix up the rough edges of StarCraft and make something even better. Even if the story isn't bad when looked at in isolation, given that it was such a decline from StarCraft 1, it was probably one of my biggest plot disappointments in ages.