There are some cool parts about this pre-order, it isn’t just a plushy toy or anything, you actually get access to the Legacy of the Void beta test realm. Heads up, your achievements and progress won’t carry over to the live game. From Battle.net:
Those who pre-purchase digitally or lock in their pre-order at select retailers will also gain entry to the ongoing Legacy of the Void beta test. Pre-purchase digitally through Battle.net now, and you’ll also gain immediate access in beta to all three “Whispers of Oblivion” prologue missions featuring the legendary Dark Templar Zeratul.
Also, anyone who purchases or pre-purchases any edition of the expansion will also unlock access to an upcoming, yet-to-be-revealed StarCraft Warrior Hero in Heroes of the Storm. If you get the Digital Deluxe Edition ($59.99 in comparison to Standard’s $39.99) you unlock a slew of things for other Blizzard games upon Legacy of the Void’s release, which I think is pretty cool.
Legacy of the Void will also be available in a retail-exclusive Collector’s Edition, which comes equipped with the digital bonus items from the Digital Deluxe Edition, as well as a special edition of the upcoming full-color hardcover StarCraft Field Manual; a DVD loaded with cinematics and special features; and a CD soundtrack.
Also well worth noting, Heart of the Swarm is now available in a stand alone version.
To pre-order, get to Battle.net.