Once you start to learn the Starcraft metagame - the competitive strategies and the micro required to execute them - the game becomes really engaging.
This is because every single action, from moment zero of the match, feels freighted with meaning. As you race to clone your SCV's mine orders and start your build order, as you send a single worker out to scout and try to figure out where the enemy base is, as you micro your scout against his workers to try to see as much of his base as possible, you can feel a tangible connection between every mouse click and your chance of winning the match.
It's a rush.