I've decided that there is, indeed, a hierarchy of difficulty levels in the different genres of writing. This may not apply to everyone, but it applies to me and probably most other writers.
Fantasy is the easiest to write. It requires the most work, but in terms of how easily the words come, it's pretty straightforward. You can do pretty much whatever the heck you want and tag it into the category of "because it's magic." Granted, you have to come up with an entire world on your own, but for me, that's actually easier. You get to start the world over from scratch. Whatever you say goes. As long as you don't break any laws of physics -- without saying it's magic -- you're good. Languages, fairy tale creatures, all are at your command.
Scifi is the second easiest. Like in fantasy, you can do pretty much whatever the heck you want and tag it into the categoy of "because it's the future." The catch is, you have to make it conform to laws of nature. In fantasy, people can fly because they're magic, gosh darn it! In scifi, people can fly because they've got rocket shoes or a jet pack and stuff. But besides that, you can do whatever.
General fiction comes next, but it's got subcategories.
Superhero stories are the easiest of the general fiction. These are the fantasy novels of the general fiction world. You've got to conform to certain rules: for example, your hero must have a weakness easily exploited by his enemies. But besides that, you can do whatever. Your villains can have whatever powers, and they don't always need explanations for how they got said powers. You can sometimes even fall back on "because it's magic, gosh darn it!"
Then come spy novels. These are the scifi novels of the general fiction world. You can do whatever you want, as long as your character has gadgets to back it up. And they can be futuristic gadgets because they're spies, so there.
And, finally, the fiction fiction. You know, the stuff that's based in real life. That stuff's hard to write because everything has to be believable while at the same time telling a story that no one's ever heard before.
Moving on from the general fiction subcategories...
Then, there's comedy. Yeah, comedy should be easy, right? It's actually not. It requires a knowledge of the human psyche, what's guaranteed to make everyone laugh and what's only funny to you. You have to balance wit with slapstick, and you have to time everything just right.
Finally, the hardest fiction to write, in my opinion, is mystery. When you write a mystery, you're essentially working backwards. You take the end -- whodunnit -- and built a plot from there. How did they do it? Why? How do the characters figure this out? How do the characters play in? Who are the red herrings? By the time you've figured all that out, you also have to figure out how all these people can have anything in common, how they all meet, and how to make it believable. Eww.
How do I stack up? I've got a spy novel, a few fantasies, and a scifi. Hardly the brilliance of an accomplished writer. But, I suppose, we've all got to start somewhere...