I'm sorry, but I disagree - it's different in some ways, but not as different, and certainly no more difficult, as some folks make it out to be.
Sorry Shaman, but I agree with Ralts here, at least when talking about a module as a product someone would pay for.
The problem with writing adventures for d20 Modern is the range of campaigns is much wider. There is no agreed upon "Tolkien/Howard/Dungeon Mag" center for d20 Modern.
You could write the best Shadowchasers/Urban Arcana/Buffy style adventure in the world and it would be completely unsuitable for over 75% of Modern games.
Similarly, an adventure involving a non-FX style military adventure, say a hostage situation, would have to be almost completely rewritten for a Buffy style game where one or more of the PCs had access to magic.
Think about the difference in tactics between a group with access to something moderately low level, say Invisibility and a group of elite Special Ops. Just the difference of a single invisibility spell makes a huge difference in terms of being able to see inside the plane.
And then of course, an adventure that took into account the Buffy style AND the no-FX style (which I contend already makes writing a Modern adventure much harder than a typical fantasy adventure) would STILL be unsuitable for a lot of campaigns.
Let's take the example of the airplane hostage assault.
Obviously the "prime" setting for such an adventure would be a no-FX special ops game.
The Buffy game would need to tweak the premise (perhaps a friendly NPC or magic item needed by the characters is on board) and would still likely need to up the difficulty to take into account the magic at the PCs' disposal.
Let's look at other popular genres the adventure would STILL be wholly unsuitable for:
Supers- major tweaks required. NPCs that came with the adventure would have to be completely rewritten into supervillains.
Also the rationale behind the adventure would likely need to be changed. Also, supers just dont generally DO hostage situations often so the reasoning behind the adventure will ALSO likely need to be changed.
In short, tweaking (rewriting) such an adventure for a group of supers would likely render it unrecognizable.
Psionics: One of the campaign models in the core rules, a group of psychic PCs would likely find a typical hostage scenario incredibly easy. Simply control their minds and have them surrender.
Again the NPCs would need to be rewritten from scratch or heaviliy modified.
X-files/Conspiracy games: Some tweaking required. Such PCs might have the negotiation skills to handle such an adventure, but an hours long negotiation is not the stuff of a typical hostage investigation. What if combat is required? A typical Call of Cthulhu type party won't have the combat muscle needed to take on heavily trained terrorists.
And of course, if the PCs have magic or psionics, see above.
This is just an example of one of the more typical adventure scenarios.
You mention some spells, like Divination or Teleport are difficult for fantasy authors as well and this is true. But the difficulties are magnified tenfold when you have to write an adventure for a group of PCs who MIGHT or MIGHT NOT have access to those spells.
Writing an adventure for 10th level Modern PCs means configuring encounters for a group of PCs who might have guns, but who might also have access to invisibility.
Up that to 15th level, and you're trying to write encounters for PCs who might have no FX, or who might be able to teleport.
THAT isnt "difficulty" its a nightmare.
Its just harder to determine the "mean campaign style" for d20 Modern than it is for D&D. And anytime you see me say "tweaking required" in the examples above, you've lost a sale if its a professional module.
People dont PAY you for an adventure where they need to reimagine the premise and rewrite all the antagonists. It would be no less trouble to write something themselves.
So like Ralts, imo writing adventure for d20 Modern is a lot tougher.