Weather
Minor niggly turbulence is common on the routes we operate - you can almost guarantee the seatbelt sign will be on as we pass overhead the Pacific Islands in particular.
Personally, the two worst incidents I've experienced were overhead the Philippines en route TPE-AKL, when the bar carts hit the roof. Luckily the flight was only about half full and noone was seriously injured.
The other was descending into CHC after a trip from NRT - passing over the Alps, we hit some clear air stuff that was unexpected, and I ended up hitting the bulkhead and dropping to the floor.
I know a couple of crew who have been less fortunate - one girl on a NAN-AKL flight had the entire contents of a fresh coffee pot soak her head and face, and to add insult to injury was then hit on the head with the silver metal pot! She was off work for a few weeks as I recall.
Another guy, same route different flight, was in the galley when the aircraft suddenly dropped a couple hundred feet, and his head smashed through the plastic light fitting and cut his face severely and he was centimetres away from losing an eye.
However, these incidents are rare and I've only ever been thrown off my feet three times. We have full discretion to be able to suspend service and sit down any time we feel unsafe.
PER flights
Yes, the demographics of the passengers is what makes it challenging, along with the operating hours of the flights.