Whereas on the piano the sound fades very quickly, organ pipes can sound as long as you hold the keys. This makes not only the depression of the keys but also the releases a crucial part of the overall performance (great pianist do this on the piano as well, by the way).
Also articulation is one of the most difficult things to achieve properly for people who are just starting out their organ studies. Knowing when to apply legato, non legato, staccato, articulate legato etc. will actually elevate your organ playing to a whole new level.
What's difficult, is to understand and to achieve the high degree to differentiation of articulation between various parts (especially in the middle parts). In order for this to happen, you have to be always asking yourself, "is this the best I can do?"
Here's the difference between a good performance and an excellent performance:
Attention to detail.