I've only flown AA a small handful of times, but my personal favourite (thus far) has been a seatmate who used a paper cup as his tobacco chew disposal -- and then left it stuck in the seat back pocket at the end of the flight!

Other than that, I'm endlessly entertained by the amount of stuff people will attempt to cram in to the overhead bins, especially the ones who try to use FC overhead bins when they're flying down the back.

Is it weird that I'm looking forward to reading what gets posted to this thread?