When you're not building and recruiting, you're following arrows and doing what the game tells you to do. 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. And even the easier moments exercise your mind in the way that only a real-time strategy game can.