When you're not building and recruiting, you're following arrows and doing what the game tells you to do. Bring this drone here. Leap on this enemy there. There are optional side-quests, yes, but they're almost always marked on your mini-map, and they're way too easy to find and finish.
But it's still StarCraft. It's still a joy to go through the now-familiar ritual of training and killing, and even the easiest missions are designed with the attention to detail that we've grown to expect from this series. Every single mission is interesting in some way. And even the easier moments exercise your mind in the way that only a real-time strategy game can. As Raynor points out, it's just like riding a bike.
Heart of the Swarm's campaign—which took me about 11 hours to finish—is quite good, and I imagine I'll play through it again at some point in the near future. There are some surprises, and most of the dialogue is so silly and endearing that it's kind of brilliant.