I can say something is easy when I can just go and do it and enjoy my time doing it. If I am sweating and making mistakes playing an Etude, I have no right saying it's 'easy'... but I can say that one day it will be.
An 'easy' piece for me that I always go back to and just play for the sheer fun of it is 'La Fille aux Cheveux de Lin'. I like playing around with that piece, bending the rules, thickening up chords, repeating sections because they are awesome. I LOVE the the Eb major section that just sounds like a jam straight out of a bluesy piece. It's incredible, and it comes and goes faster than I want it to.
A piece like that where I can just shape it any way, under any circumstance, I'd consider 'easy'. But, if someone has never seen the piece before, I can't really explain to them why it's easy, or if it will be easy for them.
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Obviously, Mary Had a Little Lam is easy. It's like typing out your name on a computer keyboard... that is EASY.