Why do some people find playing a song very easy and others find it exceedingly difficult? I think it's because as we practice we are basically trying to crack the code of a piece. As an example, look at this magic eye picture. Some will see the image immediately, some it takes a moment, and some never seem to get it.
The people who see it right away say it's a snap and can't believe the others don't get it, whereas people that don't get it often don't even believe there is a picture to be seen. Yet, everyone has eyes and unless there is some vision problem, should see the hidden image. However, not everyone will, and it's all to do with perception. Music is the same.
Sure some music is technically difficult, but it's surprising how rarely that's actually the case once you reach a certain level. It's more figuring out the code. That doesn't mean you practice less, it's more that you really dissect the piece, practice little sections at a time and do whatever it takes for you to crack it.
Back to the magic eye; people in the middle (most of us) squint, turn our heads, move closer, move back, cover one eye, ask for hints, all kinds of things to crack it. It's the same with music, and the best part of all, is once you have cracked one type, the next song in that style or genre is that much easier as you have the code partially cracked already. For that reason it's important to try many different styles of music.
Another way of looking at it is what I call the take me away feeling, and is best illustrated with reading fiction. When you first start learning to read, you sound out words and reading is a rather laborious task. As you get better, it becomes easier and you begin to anticipate words, skip over some words, and magically re-arrange content so you stop getting lost in the details. Also, when you read a novel by an author, it takes a bit to get into their way of writing. By the time you have read enough of the book, you suddenly go into that take me away feeling, as time stops and you feel like you are in the story. The more you read the faster and more complete the transition into this wonderful take me away world.
When I was in high school, I was better than average at marches, folk music, and Baroque music. Anything with a regular 4/4 or 2/4 time signature was easy for me. Waltz music was another story. I struggled mightily with 3/4 and 6/8 rhythms, and (insert dramatic music here) swing; forget about it. I joined a jazz band and found the whole experience very distressing. I had suddenly gone from star of the marching band, to incompetent fool of the jazz band.
Finally, I quit the jazz band and proceeded to avoid complicated rhythms as best I could, until one day, I played in a pit band for Man of La Mancha. The music is rife with compound times of 12/8, 7/4, and 5/8, and I was hopeless at trying to count them. Then, as if by some miracle, I discovered I had to feel the rhythms rather than trying to count them strictly. My perception changed and the windmills came crashing down. I had cracked the code for this type of music. I had seen the "hidden image," and it was glorious.
I have perhaps taken this casual approach to rhythm a bit far because now whenever I post up a video of me playing on YouTube , there are a significant number of criticisms of my playing, particularly when it comes to rests and my total denial of their existence. In my defense, I find that as with the Magic Eye pictures, if you strain too hard at the detail you never see the image, with music, if you strain too hard at the details of the rhythm, you miss the feeling of the piece.
Me playing "Colours of the Wind", notice my total disregard for what's written.
One of the best examples of this cracking the code theory is the Pink Panther Theme. It's not a particularly difficult song, once you get past all the accidentals and unusual chromatic movement. Some say it's really easy; some say it's really hard, but I think it's just a code we don't see often, as with magic eye pictures, some see it, some don't, but the more you look at them, the better you get.