Last time, I said I’d “provide short descriptions for each location on the [Minocra campaign] map.” Actually, that was a pun. What I mean to say was I’d like to share some thoughts about writing up hex encounters.
These thoughts (which are mine) apply to both Major or Minor encounters (as described in part 2 of the Hex-based Campaign Design article). Since I’m wearing my pedant hat, let’s start with the basics.
In my OCD-coloured world, an individual encounter serves at least one of three purposes:
Character advancement: Usually a combat encounter, an opportunity to grab some treasure, or both. Combat could be with a single monster type or two different kinds of monsters working in tandem. Encounters of this sort could be one-offs (think Book of Lairs) or linked into a larger adventure. Regardless, the encounter offers an opportunity to earn experience.
Player engagement: Usually a puzzle, trick, or riddle that relies on player skill to solve (as opposed to character ability). Not to sound all philosophy-game-theory-meta guy, but it’s important to keep your players interested by jogging their brain cells. These encounters let players contribute using their own knowledge, instead of just having them roll dice against stuff on their character sheet
Campaign background: Usually something the PCs can explore, investigate, or ask the setting’s inhabitants about. Instead of earning experience or problem-solving, the PCs learn about their world. In most cases, the encounter is a person, place, or thing that could benefit the PCs, if only they could unlock its secrets, learn more about it, or connect it to something else in the setting. The ensuing research reveals details about the setting that the players and GM can build upon.
The first rule of writing an encounter description is to keep it short. Short is always (and I mean every time always) better. There are about 800 reasons for this, chief among which are avoiding unnecessary work and thus reducing prep time.
To limit your focus to only the most important details, assume that your PCs will never visit this hex. Just jot down what immediately comes to mind and move onto the next encounter. Pretend each hex is queued up in the receiving line at your wedding. “Hi, Jack. How’s the spoon factory?” Next. “So glad you came, Alexa. Did you and your husband invest in that clove-mining operation?” Next. “Oh, Eustace, I’m so glad to see you’re eating solids again.” That sort of thing.
Of course, your PCs probably will visit the hex. But when they do, your short description will be all the more flexible. Now you can adjust or invent specifics in the context of the current adventure or, more broadly, the PCs’ previous encounters in the setting. Keeping the description short gives you the wiggle room you need to customise the encounter during play.
And during play is really where I’m going here. Back in my wide-eyed days of puzzling over the concept of dice with 12 sides, I wrote full-on adventure modules that covered everything. They were fun to write but useless to players. Too often, during play, I’d end up ignoring the details, realising that they weren’t helpful or even necessary from the players’ perspective. So I’d change them to mesh with what the PCs were interested in at the time, frequently inventing new ones to tie up loose ends or creating new rabbit holes to explore.
So, learn this: lengthy details are your enemy. Instead, limit yourself to 80-120 words that cover:
First impressions: What do the PCs encounter? Is it a monster, a ruin, a knight mounted on a giant snail, a boat stuck to the side of a mountain? Put another way, what do the PCs interact with?
Key features: What results from the PCs' interaction? Maybe a stand-up fight, a discovery, or an unfortunate event. Who knows? But the answer should depend on the PCs’ choices—how do they behave? will they choose greed over charity? will they help the Snail Knight or attack him? do they push the candy-like red button?
Lasting impressions: What’s memorable about the encounter? This doesn’t have to be some extraordinary, inexplicable, magical, holy-crap-you-just-summoned-Orcus! event. Maybe it’s a normal fight with opportunities for bold heroism, or a puzzle with a great reward, wondrous to behold. Or maybe it’s just a normal trap that forced Black Dungal to change his name to “Lefty.” Consider: what about this encounter would a player include in his session report?
Game mechanics: Where applicable, include game terms—die rolls, modifiers, attributes, save types, etc.—to connect the encounter to the game rules. You must include this. It not only helps the GM run the encounter, but it also helps represent outcomes in a way that impact the PCs mechanically. 
Here are a few quick examples: 
Hex #1105: The Guardian (Hills)
The hills here are patrolled by a ghostly warden known as The Guardian (actually a leather-clad spectre mounted on a double-strength phase spider). Legend says that in life, The Guardian was a bandit lord who hid his spoils in small caches throughout the hills. How he died is unknown, but his attachment to gold prevented him from passing into the next world. Rumours abound that a map to his treasure stores is clutched in his dead hands, if only his body could be found.
Hex #0819: The Weeping Idol (Jungle)
A 16' stone idol of a demonic, spine-headed humanoid stands covered with vines. Its white opal eyes stare blankly ahead, but if approached by a magic-user, the idol begins to weep as deep azure pupils appear in the opals and follow the mage's movement. If the mage attempts to provide comfort or assuage the idol's sorrow, he gains the idol's sight for 1d4 days. Any attempt to remove the eyes causes the offender to weep constantly (CHA -2) until treated with Remove Curse.
Hex #1522: The Stone of Madness (Underdark)
A lone stalagmite stands in the centre of a broad and frequently trafficked Underdark tunnel. As the sole subterranean feature for miles, the stalagmite is the centuries-old target of Speak with Stone spells cast by travellers of all races, eras, and dispositions. Such diverse exposure, added to the lack of any natural sentience, has imposed a tinge of madness and diminished the rock's capacity for coherent conversation. Consequently, there is a 3-in-6 chance that it provides non sequitur answers to questions posed (on a roll of 1-in-6, the stone behaves as if paranoid and offers deliberately false information).
Coincidentally, Joe over at Inkwell Ideas, best known as the author of Hexographer, is holding a Hex Crawl Contest. He's taking 12 submissions of eight hex encounter descriptions each by March 31, 2012 for a free copy of Hexographer Pro. At the time of this writing, Joe’s received six of the 12 entries, so make haste!
OK, per usual, that took longer than expected. It is possible that I rambled a bit. But chances are you’ve overlooked that if you read this far. So, cheers for that.
If it’s not clear, all of the above reflects my preferences for creating and running a campaign. YMMV. That said, how do you write hex encounters? Why not include one in the comments?
The one exception to this rule is monster stats. Don’t include them, or rather, don’t include a full stat block. Primary reason is because it takes time, even if you’re just cutting and pasting from a sourcebook. Second, remember my advice about adding a lot of detail: don’t add a lot of detail. Stat blocks count as detail. What if, at runtime, you want to replace the orc guards with goblins? Shame on you for statting out orcs ahead of time. If you do have something specific in mind (like a really strong ogre), just translate it mechanically in the description (e.g., the ogre gets +2 damage due to strength).
The wise and ancient among you will discern hints of Judges Guild throughout. I make no apologies.