In my opinion,
Most of the flying I have done has been above 7,300 FTAMSL My home airport is at 7,340 and my other base is at 8,445, whenever I fly to the coast nothing seems difficult. You get so accustomed to having reduced performance in the hot and heavy enviorement that you get this grin on your face whenever you pull power at sea level, everything seems like a piece of cake.
Then on the other side pilots who are accustomed to SL ops. have to go thru a whole new learning process when they first come up here, and sometimes this learning process involves some "interesting" unexpected surprises.
Flying at altitude demands more planning, technique and skill which can be gained thru experience if you get a chance to learn in such conditions its always a good thing, though not a must.