My first observation actually started at Marine Corps Marathon last year but then occurred again at Rock and Roll Arizona. Runners lining up in an inappropriate corral. Unfortunately this gets the "slower" run/walkers (which I consider myself) a bad name. Both of these races had honor system for entering the corrals except I assume for the elite runners.
Unfortunately because of slower runners lining up at the front of the corrals bottlenecks in the early miles occur. This creates challenges (and even hostility) to occur for everyone. Even worse is when 3 or 4 competitors will abruptly stop and start walking in a pack rather than single file. I completely support every runner whose goal it is to run a marathon; however, it is unfortunate when common courtesies are not implemented. I admire and respect race directors who remind runners to place themselves in corrals that are appropriate for a reasonable guess of predicted finish time, pay attention to other runners, walk on the right (same side as where slow cars should go) and give a heads up when runners plan to walk.
My second observation has to do with passing particularly in single track events. I have run numerous trail ultras and have not had any issues (and hope I have always been respectful). I indicate if I will be passing on the left or right and ask those passing me which side I should move towards. If I hear a pack of runners I tend to move off and standstill particularly when their approach is quick and imminent.
Unfortunately I recently was in an event in which many times when I came up behind runners and tried to pass, the other runner would speed up. All this would mean is I was on a nasty surface for a longer time period. I understand keeping a steady speed but actively speeding up (particularly when the passing terrain is treacherous) seems unsportsmanlike. If you realize that you have been slowing down, fall in behind the runner, draft a bit and re-pass. But speeding up, especially if you slow down once you have been passed doesn't seem productive.
My final observation is the assertion that runners with headsets can/do hear others. I have numerous times indicated, "on your right", "excuse me" or otherwise made a statement or asked a question that deserves a response (verbal or through movement). Numerous times I have not been heard or acknowledged. This actually caused a medical response to be delayed in front of me at the Disney Marathon. Even with the "sirens" on the EMT bicycles runners with headphones on appeared unaware, oblivious and unresponsive when bicyclists were going to assist an injured runner. This happened for a significant distance as the bicyclists worked their way forward.
This is unacceptable.
I would not want to have a medical issue in which every second mattered and because of runners not responding to clear warning signals permanent damage occurred, nor do I imagine would any runner want this for themselves or a loved one.