What's the most difficult thing to learn about organ playing? ...
Personal bias can play a big part in answering this question, depending upon whom you ask; for as many people as you line up, you're going to get just as many different answers depending upon how much difficulty they may have personally encountered with it in the beginning.
Someone might insist, hands down, that it's the touch.
Someone else might say that it's the fingering and hand division.
Others might swear that it's got to do with sight reading and transposition, or maybe improvisation.
A strong argument might be presented that the hardest thing about it is learning to get over the fear of the instrument, the fear of making a mistake, and the fear of playing it in public; but, then again, there are at least an equal number of players who aren't afraid of it at all and consider elements of pure technique more challenging, to them, than controlling their fears.
In a 4 part texture such as hymns, when moving to the organ from the piano, the hardest thing to overcome is getting the bass line out of the pinky of the left hand and sending it to the feet, then getting the left hand to work the tenor line all by itself.
Between our feet and left hand it's like we're all a little dyslexic in this respect, at least in the beginning; we read the bass line on the page but the instructions are carried out by the left hand instead of the feet; then we have to mentally associate parts with hands and feet to continue.
We can start out with having a fairly good background in piano playing and then along comes the organ with pedals, and now we're faced with wanting to double the bass line with the left hand!
People who come to the organ without any piano background can notice the opposite problem; at some point, as they continue to grow as an organist and discover Bach and all the other rich literature for the instrument that has them capitivated, and the desire to play it becomes almost overwhelming, they realize that they're going to need to come to terms with the piano to get where they want to go with the organ (it's like hitting a brick wall), and, when they move to the piano, associating the bass line with the pinky of their left hand is something new for them; some of them might encounter consider trouble with it at first.
In both cases, it's the same neurological explanation:
The pedal part, as far as our brain is concerned, is like a third hand that expects the brain to develop new neural pathways that are not there; it's like learning to ride a bicycle, at first we stumble, we fall, we trip, but eventually we get better with practice; after that we still stumble, fall, and trip, but less with more practice.
The more neural pathways we have in our brain, the faster we're able to get the left hand and feet coordination going.
It's a problem that in the beginning just about all of us have more or less success trying to eradicate; very few can escape it.
And since the majority of people are right-handed, this might also have something to do with it.
This may be easier in the beginning for left handed people, but that's just a guess; I don't think anyone really knows.
Everyone is different ... the adjustment seems to be very easy for some, they might not even encounter the problem, but for others it might take a long time to make it work.
All of this is normal.
Objectively then, in terms of playing its music, the absolute hardest thing about learning to play this instrument -- the thing that would probably get the most votes -- is overcoming the challenge of getting the left hand and feet (photo) to coordinate, to move independently and exactly in time with the right hand ... a challenge made more challenging by continuing to indulge in the habit of doubling the bass with the left hand when playing 4 part hymns.
This habit leads to trouble later on when trying to play repertoire with independent parts for the left hand and feet.
How well an organist has met that challenge can be tested by playing in a 3 part texture where there are never more than 3 notes sounding, simultaneously.
This can have the player on a bit of a slippery slope, trying to get all 3 moving lines to sound exactly in time, as any error in timing, no matter how slight, will be heard clearly.
Organists have coined the term "cow on ice" to describe the situation when they're trying to play a trio like this [See menu bar, Free Stuff, Trio in Eb Major Op. 14].
The important thing to remember here is not to get intimidated, to first practice each line separately, one at a time (right hand, left hand, pedal), then practice 2 lines at a time (right hand and pedal, left hand and pedal, both hands) before trying to put all 3 together [See blog, Getting Started With Writing, Part IX].
Whenever difficulty is encountered playing trios like this it's almost always due to not practicing each line separately, then 2 at a time, at a slow tempo.
By practicing the manual parts together for the right and left hand an equal number of times, the left hand part will always be weaker.
The left hand and pedal part often need to be practiced more than the right hand and pedal part, and it's a good idea, when learning a new piece, to start working on this first.
If the left hand part is tricky it should be practiced enough so that mistakes don't happen; typically, this is more than one time through without making a mistake.
Beginners tend to practice until they can get it right one time, then stop; actually, we need to keep practicing it until we can play it several times without mistake, then memorize it.
Since the left hand is typically weaker than the right hand, it will also help to practice the manual parts of organ pieces on a piano or an electronic stage piano which has the actual touch and feel of a real piano.
Habitual practice on electronic keyboards and pipe organs with electropneumatic actions typically won't develop the left hand very well, to bring it up to speed with the right hand.
For an organist, it helps to practice Czerny (pronounced "chair'-knee") etudes on the piano to help develop the left hand and build equal strength in both hands.
Carl Czerny was a piano pupil of Clementi, Hummel, and Beethoven and a prolific composer who wrote tons of studies for the piano (over a thousand works), most of which have passed into oblivion today, but some of his works have survived and are still recommended by piano teachers [See blog, What About The Piano, Part IV].
It helps to work from a copy of Czerny's "The Art of Finger Dexterity Op. 740."
There will be an improvement in finger strength, especially in the left hand, by working just the first etude (in C Major) from this book, which isn't hard to read or to learn.