For me, it's familiarity.
I worked on four Rachmaninoff Preludes this year, and the four pieces Op.119 by Brahms. Generally, the Rachmaninoff would appear to be more difficult (D major, Bb major, G minor, B minor), but in all honesty I was MUCH less used to Brahms' harmonic language that the first two pieces in Op.119 caused me quite some grief when working out voicings. Because I have already played a bunch of Rach, the LH at the opening of the Bb prelude, and the big heavy octave/chord stuff in the other preludes actually seemed easier to approach and practice.
When I took the b minor Klavierstücke... it was a mess of harmonic analysis, finding voicings, and a lot of struggles with pedalling! It was not nearly as technical as the Rachmaninoff, but I have gotten to a point where technical difficulties are a lot more straightforward than musical difficulties. If a passage does not work at a fast tempo, I work out the fingerings, do some rhythmic practice, and keep it slow for a while so I can adapt. If there is a passage where I am asking myself "what the hell is this supposed to sound like?", and after hearing different recordings, you hear people play it all differently, I find it leaves me with so much more work to do.
I find technical challenges more exhilarating to practice because I can see steady progression, so I find pieces with these elements a little easier. Pair this idea with Debussy/Rachmaninoff and I'm good to go!
I'm going to try working on Op.118 this summer... I can tell it's going to be mostly work on interpretation, which I really do not find easy!
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Anyways, I kind of link 'easy' with "familiar" and "something you can work at progressively". Going to the extreme, if I was asked to play some Schoenberg, I would have a VERY hard time finding a coherent way of working it out and really getting to the root of the piece. So 'familiarity' kind of goes along what type of harmonic language you are used to, and also (in the case of Liszt/Rachmaninoff) what pianistic feats you have already conquered - so when they come up again, there is the mentality of "Oh, I've already seen this before!".
It's 2AM here, and I was actually thinking about this earlier today (just finished my recital, and am starting some new rep soon!); I was flipping through books, and just found it interesting what I considered to be 'easy' versus what looked 'hard' (and most of the time I was disregarding sheer technical difficulty).