I'm one of the most courteous riders you'll meet on any track.
If it is a taped-out DH track and there is a person walking their dog down it, I will stop and explain how what he is doing could be dangerous. If I am riding down a walking track and see some people walking, I slow right down, let them pass with a "hello, nice day" kinda attitude.
There are enough stupid w*nkers out there who are rude enough to ruin it for everyone with their 'I'm f*cken awesome' attitudes. You see that a lot here in the Alps.
As for the original posting, I heard of someone being practically decapitated by a line of wire stretched at head-height. I think it was a moto rider, but the horror of trail trapping remains. Anyone caught trapping trails should be dealt to severely.
I'm like that most times. For the most part hikers in AZ are good and will let you pass if it's a difficult and narrow trail. On occasion there are groups trying to provoke problems.
There is a club called AZ roadrunners(?), they run cross country on trails, and consider they have the right of way over everything and everyone. The run side by side and won't let a rider pass, they frequently run in with MP3 players, and if alone swerve all over the trail refusing to let anyone pass.
I've even had them step out of the bushes directly in front of me on a rocky downhill and yell "I have right of way". Sure you do, you also have the right to be splattered with baby head rock as I go around you, because you are sanding on the only clear part of the trail, and are a wad too boot.