A couple of weeks ago the park district did on-site surveys of the MUP trail users. I spent some time visiting with the person who oversees the trails and was heading up the survey. I asked him what they found to be the biggest complaints. He said the most frequent comment concerned "high-speed" bikers. More people complained about being endangered by this than any other issue. A secondary complaint was just lack of trail etiquete - the typical issues we all encounter on the trails.
I was not surprised by his comments. The only times I have been concerned about my safety on the trails have been caused by some hotdog in spandex riding like they were training for the T de F. Weaving through traffic at high speed, taking blind corners on the wrong side of the trail, and as the OP indicated, speeding past without announcing. I have also had occasion to help pick up two or three of them when they overcooked a tight corner on their carbon bike and dumped it into the trees.
Riding at a speed that is appropriate to a MUP trail, I can usually dodge a wandering jogger/dog walker/family groups, etc. but it is hard to dodge a biker doing 20+ mph in the wrong lane on a blind curve.
But the most unique situation I have encountered on the trails was last Sunday. A girl in her early teens had spread out a beach towel on one lane and was laying there in the sun getting a tan.
The park district posts trail rules but I have never seen any enforcement, so the random craziness continues.