I think it's about walking that thin line between having the direction but not the road map.
I used to be an obsessive plotter, now I'm more of what I call a 'connect-the-dots' person - pretty much what a few of you (like Phyre) have said above. I map out key plot points (the 'dots') but leave the in-between bits pretty loose so I can go off-road as much as I want. Of course, some scenes - like action sequences - need step-by-step plotting, but they're so fun to write that I find that doesn't spoil it for me at all.