I didn't write them down, so I am working from a distant memory.
1. R/T, to be expected though. Many came over prepared as best they could. Some had purchased tapes of US RT techniques, or somehow prepared themselves in this aspect. Alot hadn't. At first it was quaint to hear them ask for curcuits.
2. Problems in high density controlled airports. This encompasses a few things. Speed adjustments issued by controllers. At a number of San Diego airports the pace can be breathtaking. Instrument pilots especially had problems flying at the increased speeds ATC issued, an ILS at 120 kts instead of 90 or whatever they were comfortable with. VFR patterns adjusted by the tower offered the same.
3. A number had problems at a busy uncontrolled airport. A bit of a problem being their own controller.
4. The complex airspace of Southern California. Enough said on that.
5. Flight in reduced vsby, as is the norm here.
I recognize some of these are particular to the area, I based my judgements on the ability to adapt to these challenges after some exposure, not how they came through the front door initially.