"What Kerrigan Did Between BW and 2"
I was never satisfied with the "Kerrigan did jack diddly squat during the interregnum". However, I do recognize that for SC2 to make sense, evolution has to occur.
As I understood it, the Zerg, although their build structures are based on "evolutions," in the original lore they were parasites. They did not create "hydralisks" from scratch but instead harvested the dna from a pre-existing creature. So, for new evolutions to occur, I saw that there had to be some new impetus for the evolutions.
In many ways, the same can be said for Protoss and Terran advancements: these are technologies that would take decades to research in most conventional narratives. For them to occur and become implemented in 4 years is both impressive and a bit jarring, at least in my mind.
That leads to the essential conceit of the game.
"What is 'The Will of Atai'?"
As the last mission reveals, the objective of Atai--prisoner of the Xel'naga--is the annihilation of all three races of the Xel'naga creations. As such, it is his intervention as much as it is the crystals of Boccob-II, that leads to the dramatic evolutions in combat technology for the three races during the 4-year period.
He has the technology (being on an equivalent technological and intelligence level as the original Xel'naga), and the desire. As such, he grants Kane (through the reformed body of Jiinsh) the Immortal technology. He grants Kerrigan the "Queen" replacement for Cerebrates with the eggs of Zakat. And he grants the Terrans the technology of the Crucio tanks etc. with his creations on Boccob-II.
And, what I really loved about the concept of this villain, is that in its context, the events of Starcraft 2 very likely will be following "The Will of Atai" -- both literally and metaphorically, since the three races will once again in all likelihood be destroying themselves, and thus achieving his objectives.
Thus, 'The Will of Atai' hoped to answer the following major questions:
1) Where did the new technology for all three races come from?
2) What did the Zerg do during the intervening years [Once Kerrigan decimates Argeth, it provides ample grounds to test and evolve her new forces]
And as a bonus it also helped to reinforce Raynor's decent into alcoholism and failure (being once again 'defeated' by Kerrigan, in that she cuts off his source of income after he expands significant forces to gain payment).
I hope you all enjoy the campaign!