An example happened to a group of us on a rare kind of day in February. A photographer in a particularly bright outfit chose to dwell in a shot for at least 20 minutes, to the great frustration of serious photographers waiting for him to leave. I decided to make the shot anyway and choose later whether to clone the one person out, which is easier than a large group.http://img.photobucket.com/albums/v467/bil...3-0839-w720.jpg
A figure adds scale. A crowd detracts. I'm not sure about this one. I'll probably end up cloning the dolt out.
Was he there before you. I certainly agree to walk it's rude to somebody else's shot (although we've all probably done so unknowingly at least once, so a polite request may be all it takes to resolve the situation).
On the other hand I think the first one there has the right to set up where he/she wants, so for somebody else to come along later and complain as in grepmat's example seems unreasonable to me.
If I were there first I wouldn't feel obliged to move just because somebody was setting up behind me. I'd try to avoid lingering any longer than necessary, but I wouldn't allow myself to be rushed and risk not getting the shot I was there for.
And let's face it, if shooting digital sometimes there are simple solutions to these types of problems. Depending on where the person is it may be a simple matter to clone them out. I've also had occasions where I was shooting a scene that was never completely free of people as they steadily walked through the area, so I "bracketed" multiple shots to give myself the option of using layers/masks to produce an end-result without any people in it. (Some pedantic purists my feel this is cheating but I don't feel the least bit guilty about using such techniques).