An oldie but a goodie: http://blogs.msdn.com/chris_pratley/archive/2004/02/09/70474.aspx
The best bits (in one paragraph, but split out for emphasis - bold is mine):
[...] one way to describe PMs is that they not only "pick up and run with the ball, they go find the ball". That really defines the difference between "knowing what to do and doing it", and "not knowing what to do, but using your own wits to decide what to do, then doing it". That means as a PM you are constantly strategizing and rethinking what is going on to find out if there is something you are missing, or the team is missing.
You’re also constantly deciding what is important to do, and whether action needs to be taken. The number of such decisions is staggering. I might make 50 a day, sometimes more than 100 or even 200. Most of the time there is not nearly enough hard data to say for certain what to do, and in any case almost all these decisions could never have hard data anyway - you need to apply concentration and clear thinking.