Watching your backcast. Whilst important in high traffic areas, every party needs to be aware of their surroundings. This INCLUDES pedestrians walking/jogging on paths. For a flyfisher, the back cast is exactly that, at his back. His target area is out front, and watching the water can be the difference between catching fish and not.
Personally we as fly fishers are responsible for the first lift off of the water. In other words, on our first false cast we are too ensure no one is behind us. Once it is safe, and the cast has been started, the responsibility moves to those people who are moving through the area. It is common sense to not walk behind a fly fisher until his line is down on the water (if the fly fisher puts the line on the water and lifts again, that is his responsibility) just as it is common sense not to walk infront of a vehicle travelling at speed. (This being said this does not give the fly fisher an excuse to sit there for a minute casting)
Unfortunately common sense is getting increasingly more rare these days...