I’m not going to summarize Day9’s comments or content here. That’s not the point of this post. What I do want to note, though, is the essence of what Day9 was getting at in the epic 4 part daily.
Throughout the process of talking through organization and execution of builds by several players, what Day9 points out in almost every case is the importance of scouting and knowing what the opponent is up to. For example, several times in the Daily he stresses that ‘this build works best against a fast-expanding Protoss, so if you check and see no expansions, [forget the build order and] do whatever you can do stay alive.
In other cases, he says outright that he would have preferred the player to have done more ‘checking’. In other words, knowing what the opponent is up to is the most important thing, not just for to make a point for this daily, but for me, in the whole game.
Starcraft is a reactionary game. Knowing what to do when faced with different situations works in tandem with taking the initiative and being aggressive. By constantly scouting your opponents, you will gradually figure out, intuitively, what unit composition to get.
For example, as a Protoss player, if I see mass roach or mass marauder coming, forget everything! Chrono boost out as many immortals as you can! If you haven’t got a Robotics facility, get one, and get immortals, because roaches are strong against gateway units, and marauders even more so.
Plus, knowing what your opponent is doing through scouting gives you confidence in your own build and makes you feel in control. So scout often, and win more games!