I echo all the sentiments here. Hazards abound, usually the result of ignorance and/or discourteousness.
Doublewide baby carriages? The bane of my existence, especially when mom/nanny refuses to move to the right. Or when there's a dog on a long/flexileash and/or a cell phone involved. There are a few double-doublewides--two ladies walking next to each other, each w/a two kids on board---who also refuse to go single file. Or people with little kids who let them wander all over the trail and then give you the evil eye when you politely ask the child to stay put so you can pass.
Where I ride, I encounter the most hostility from other females. In addition to the baby carriage crowd, chicks running/walking two or three abreast who refuse to move to the right, even when there's oncoming traffic in the opposite lane. They are usually chitterchattering away.