Why is this considered the most difficult nocturne? I've seen it compared to the etudes in difficulty. I've looked at the left hand from a picture of the score online. It doesn't look THAT hard, but I already know I'm completely wrong. What am I missing?

The first section doesn't present any immediate challenges but things start getting difficult in the chorale section. It is quite difficult to create a melodic line in the section with the large chords in both hands including the rolled ones. Try to make that melody sing and you will see that it is not easy.

Octave passages in the transition - okay, with some work and technical ability these aren't that difficult but again you want to continue to feel the overall melodic arch in that section. Musically I think its also very difficult to pull off.

The last section is what makes it over the top difficult 'dopplio movement' - if you take it literally this means double the tempo of the first section. Where to start on this section? 4 against 3 - again if you are familiar with this kind of rhythm maybe its not too bad. The left hand - lots of leaps from lower bass octaves to triplet chord accompaniment. It is extremely difficult to execute these at speed and ensuring there is balance between the two hands. The right hand - repeated triplet chords plus melody notes. IMO this is the most difficult of all to achieve effectively. The melody needs to sing as it does in the first part but try to achieve that while playing repeated triplet chords with the same hand!

I've been struggling with this piece off and on for a while now. I still can't get that last section up to tempo although I think I'm projecting the melody well enough. For the chorale section Chopin's Etude Op. 10, No. 11 is good preparation. Op. 10, No. 2 is good preparation for the Dopplio section because it works 3, 4, 5 fingers.

I wouldn't make absolute statements about relative difficulty. There may well be people who find, for example, the G major or D flat major Nocturne harder. Different things are hard for different pianists. Personally, I didn't find the last section of 48/1 unbelievably difficult.

Ahh...Going to be one of those people! Kidding!

For one it is a bit different then the other nocturnes, and we see Chopin use textures in a manner that is akin to Franz Liszt. Then the difficulties mentioned in the last section are a bear as mentioned in a previous post. One of my teachers compared this nocturne to be closer to a ballade then the nocturnes. The last section, with the speed, and the size of the chords is very difficult to balance, especially when you consider that you are playing the melody with the pinky.

Also, this is just my opinion, it is hard keeping this piece with the character of a nocturne, and not turning it into a cheap technical show off.

Octaves in central section + Doppio movimento section + musical difficulty

You have to try it all (not only look at the first page) because things get more difficult later. This is also a piece where hand size matters.