They made a lot of good little changes to the campaign mode and I got trapped in the “Just One More Level” mindset. It’s pretty genius even if there are all sort of similarities between it and Warhammer 40K and even Mass Effect to some degree.
They made a (semi) RPG RTS with their single player campaign. Each crew member has a bit piece to say here or there and that leads you to a mission. You get credits (money) for completing missions which you can use to upgrade your units, vehicles or buildings. Also there is a Alien Tech Tree for Protoss and Zerg that lets you pick between one ability and the other (on each of the trees) as you discover more and more about the different alien races.
I won’t get into too much detail but, good stuff Blizzard.