Probably spriting. Simply because creating proper spritesheets while maintaining a reasonably detailed level often takes ages. Many consider it boring (myself included) and actually forcing yourself to working on it is the real pain.
Hex is hard depending on its application. Doing the little overrides are fairly easy once you understand what you are doing. It's just being declared as "hard" because the ASM-code looks rather cryptic compared to what one is used to from higher-level programming-languages. The larger projects are, of course, pretty mindblowing. As an example, let's take Rammichael's hex-edit where he increased the object-count from 100 to 1000 that can be loaded. Technically, it's just an enlargement of an array, but applying it isn't that easy
Datachanging.... well, what can I say? I myself have started out with DCing, thought it was hard at the beginning. The thing that is actually hard about it is transforming your ideas into lf2-readable frames. No direct conditional statements, loops, functions, etc. That is what makes the advanced-DCing what it is: ununderstandable for DCing-newbies. But once you get behind this whole concept, it doesn't take you more than a bit of patience in achieving the effect that you want.
It is probably a different thing when creating mods, though. A full LF2-mod consists of roughly 30 characters, another 50-100 files related to them, 10 weapons, and 10 backgrounds. It's not a single discipline that makes it hard; it's the fact that it's so much to do. The sheer workload to do to achieve a unique mod is pretty intimidating. This is also the reason why, many times, people group together or need many years to split the work down to a reasonable level.
Some have the time, yet not the will. Some have the will, yet not the time. And the few that remain are the ones that usually produce mods.