For me, the things that were or are 'hard' to learn, are things that I have been able to 'hit' for quite a while (atleast 6 months) but still can only hit by luck, despite hours of schooling, these include:
Swirls/rev swirls (mainly swirls).
Torques!
Any gyro move (but spinning is easy!)
Flipside atomic (randomly just came right one day though
The one basic that I have struggled in getting to grips with is osis, I still can't do it right and my osis is less consistant than my blizzard