Second-order thinkers take into account a lot of what we put into our decision journals. Things like 1) what are the key variables and how do they interact?, 2) where is the leverage?, and 3) if I take this action, what happens next?
First-order thinkers look for things that are simple, easy, and defendable. They fail to realize that they are dealing with complex systems, or if they do realize it, they mistake cause-and-effect relationships. They are incapable of thinking in terms of second and subsequent steps. And, more fundamentally, they are generally unaware of the need to think in terms of second steps.