Ideally one running a marathon wants there to be enough people running nearby so that she is not alone for an extended stretch but not so many that running comfortably is difficult.
The question: How crowded -- too many, too few, just right -- have the marathons you've run been? Not overall, but where you were running; so we need the pace. I'm particularly curious as to where in the big city marathons the pace is such that there are too many people.
For me: 2006 NYC, 2:48: just right.