The toilet flusher
Let's start with the worst offender: The flush mechanism on a toilet is the number one dirtiest place in the bathroom. Think about it -- you go about your business, wipe, reassemble yourself and your clothing, and then reach around to flush the toilet. With that same hand you just used to wipe your rear end. Yeah. Next time, consider using your foot.
Then consider all the other shoes that have been used to flush that toilet.
The stall walls
If you're familiar with the term “aerosol effect,” then you already know why the interior walls of public restroom stalls are covered in literal shit. If you don't, oh boy, you're in for a treat! The aerosol effect, also known as toilet plume, describes the particles that become airborne when a toilet is flushed. Yup, it's flying shit! Teeny, tiny shit particles, but shit nonetheless, which lands everywhere and means that the walls, which are probably never cleaned, are coated in poop.
The stall door handle
This is probably a little more obvious, but well worth pointing out anyway. The lock and/or handle on the stall door is another place that comes in constant contact with unwashed, germy hands.