Horse feces everywhere. Some horses startled by the bikers and hikers. Mountain bikers going extremely fast (most trails are fast for them) and can easily hit you, your dog, your child, etc. And seems like quite a few don't exactly have the skill to be going that fast.
Ugly powerlines go across the main part of these trails and many of the trails require you to walk under the powerlines for a long time or else take a much longer route (all that snap, crackle, pop isn't comforting). Ruins the view much of the time.
Also seems to be popular with some bowhunters who are too lazy to go out into a more appropriate and less populated area to do their thing.
Quite a few casual hikers in everyday clothes come here with a minivan full of kids who know nothing about trail etiquette, occupy the entire trail so nobody can pass, don't maintain control of their kids, and make for an unpleasant mix with many of the above factors mentioned.
And lastly, too many superficially friendly people here - do you realize that if you make it your policy to say "hello" to everyone that this will involve upwards of a 100 or so hellos on a decent hike here? Do you people do this at Disneyland or at the State Fair too? Sheez.