Microtrash is always a big problem, and rare is the trip I take without coming home with a cargo-pants pocket full of it.  Common offenders are cigarette butts, corners of Clif/Snickers/Nature Valley bars, crumbs of those ubiquitous blue foam pads, and the occasional bandaid.  Note, I never pick up toilet paper, so ladies, please try and pee farther away than right next to the trail.

Macrotrash is a little more of an interesting dilemma.   You see either modern dumpsites at trailheads (tires, beer cans) or you find ‘historic’ items rusting away at an old cabin site, or the remains of a can dump at an old shelter.   Sometimes its piles of trash, and unsightly, or its in high traffic areas where it encourages more trash.   Here’s a case where a very busy caretaker in the Maine Applachian Trail Club cleaned up a massive amount of really bad trash.   I helped carry down some big rusty iron bits on my way down the Firewarden’s Trail (totally overwhelming my pack’s suspension, but worth it).
http://www.matc.org/assets/Resolved-Refuse-Removal-ATJourneys-MarApr2014.pdf

I can live with some of it, things like this I found in the desert of southern California.   It adds a certain atmosphere, and has some historical value.   This area was actively mined for gold.

But what kills me is the modern “woodsman’s” trash.  Plastic garbage bags left behind.   Propane cannisters.   Toilet paper.  5 gallon buckets of poop.   Benches, tables, chairs hacked together with local lumber and plywood.   When I was a caretaker on the AT in Maine, I once packed out nearly a full black trashbag, just from a single hike ~40 miles and hitting 3 shelters.  Unacceptable!

These are the things that say “This area is actively used.   It is not wild, whatever you thought you were going to see.   It’s a campsite, a playground, a city park.   It’s to be used and consumed, not preserved or saved for the next person.   We beat you to it, so suck it.”