http://www.vox.com/2014/5/20/5732208/the-green-lantern-theory-of-the-presidency-explained
Summary: The President doesn't have all that much freedom to control the government. The office was designed that way.
I agree that there is very little direct power. However, the President has a lot of power in picking the executives running various governmental departments. And staffing them with competent administrators and not political appointees is the most important way the President can influence decision making without actually having to make the day-to-day decisions. This is a pretty standard advice to middle and upper management. Obama failed miserably in this. Sibelius and Napolitano are classic examples of obvious incompetence, I'm sure you can name many more....
What is something you used to believe, preferably something concrete with direct or implied predictions, that you now know was dead wrong. Was your belief rational given what you knew and could know back then, or was it irrational, and why?
Edit: I feel like some of these are getting a bit glib and political. Please try to explain what false assumptions or biases were underlying your beliefs - be introspective - this is LW after all.