I've never liked deserts. Even when I was a kid, I found stories which take place in deserts to be pretty boring. The character is thirsty, the character is hot, it's cold at night, boom. Deserts. Of course, I'm aware that deserts have their own forms of diversity. Geological formations in deserts can be quite mysterious and beautiful, and the types of creatures with the ability to survive in such harsh environments are often fascinating to learn about. Despite that, I still don't find deserts all that enjoyable. It's a bias of mine. Cold deserts are a little more interesting to me, but those will not be covered in this post.
But you can't just send your PCs through forests all the time. Sure there are idyllic forests, creepy forests, dense forests, sparse forests, coniferous forests and deciduous forests, forested mountains, and forested planes. But for some reason, players eventually get tired of trees, and you need to throw in something else. Eventually that something else ought to be a desert. In all likelihood, since we are playing Pathfinder, that desert is going to produce a creature which is going to try to kill your players. And unless you're fine with all of your desert combat occurring within a featureless box with a desert painted on the background, you'll need to find some way for the environment to become a factor in combat.
The problem is that in many ways, deserts are featureless boxes. The box just extends further than the eye can see in every direction. None the less, there are a number of ways in which the players can use the environment to their advantage, and ways in which the environment can betray them.
Blinding Attack If movies have taught us anything about sand, it's that sand makes for a good quick blinding attack. (if movies have taught us a another thing about sand, it's that people are softer than it.) If players want to throw sand in their opponent's eyes, you can treat it as a ranged touch attack (range of 5ft.) which can be performed from a prone position. If the target is wearing any kind of full eye covering, the attack fails. Success causes the target to be dazed for one round. Bear in mind, as GM, that most creatures which evolved in a a desert will have better protection against this kind of attack than most humans would, and may not be susceptible to it.
Blinding Wind Deserts are often very windy, and that wind tends to pick up sand and carry it around the battlefield. This effectively functions as the blinding attack noted above, but against all players facing a certain direction. How often this effect occurs is entirely dependent on what the whether is like. If the characters are fighting during a sandstorm, then any time they open they're eyes they're going to get hit by the dazed effect. More likely, though, players will need to deal with either a consistent wind, or a sputtering breeze.
--Consistent Wind: The wind is coming from a single direction. If this wind is not present and defined before combat begins, roll 1d8. A result of 1 means the wind is coming from the side of the battlemat opposite your position at the table, or "north." A result of 2 means the wind is coming from the "top right" corner of the battlemat, or "north east." Results continue in a clockwise fashion around the battlemat. The GM may, at any time, roll 1d6. A result of 1-2 means the wind shifts one "space" in a clockwise direction (West becomes South West, East becomes North East, etc). A result of 3-4 means the wind remains the same. A result of 5-6 means the wind shifts one space counter clockwise. Any players facing against the wind without eye protection must succeed on a reflex save DC: 16 or become dazed for one round. Players facing a direction which is only one space off from the wind direction are allowed a reflex save DC: 13 to avoid becoming dazed for one round. Note that since Pathfinder does not include a facing mechanic, facing must be determined based on the direction in which the player performed their last action. If the player wishes to turn in their space, treat it as a swift action which provokes an attack of opportunity to any enemy the player turns their back towards.
--Sputtering Breeze At the start of each round the GM rolls to see if there will be a gust of wind this round. There is a base 20% chance (which can be raised or lowered depending on the current weather patterns, but should never be above 50%) that the battlefield will be affected by a gust of wind this turn. If there is a gust, roll 1d8 to determine which direction the wind is coming from as noted above. The effects of the breeze are as noted above under Consistent Wind.
Poor Footing Sand makes for poor footing, particularly when moving up and down the sides of dunes. Any characters not intimately familiar with desert environments suffer a -2 penalty to their CMB and CMD for any combat maneuver which requires them to have firm footing (resisting a bull rush, tripping, charging.) This penalty rises to -4 if the character is standing on a particularly loose area of sand, such as the steep side of a dune. In such an instance, the character also loses their dexterity bonus to AC.
Gets Everywhere Sand gets blown around so much, and is so fine, that it can get into anything. And if enough of it gets into a mechanical device, odds are that device will stop working. If your game includes guns, these would be particularly susceptible to this danger. But any number of devices which rely on moving parts could be at risk. Clocks, locks, or even crossbows! Players can avoid this trouble by wrapping their equipment and keeping it protected, but if they fail to do so, then they risk a 5% failure chance in the first two days, a 10% failure chance for the rest of the first week, a 20% chance the second week, a 30% chance the third week, and so on. For guns, these numbers would likely double. Fortunately for bowers, crossbows are relatively simple, and a standard action to clean out the sand in the triggering mechanism should be enough to restore the device to working order.
Just Beneath the Surface The wind over a sandy desert covers up anything quickly. It would be essentially impossible to actually track something through the sand. It would be be similarly impossible to see anything buried just beneath the sand. If foes are springing a trap on the PCs, they could easily have tripwires beneath the sand which would be impossible to detect. Weapons could also be hidden just under the surface of the sand to give the appearance that a group was unarmed. Sand could even hide a trap door with ease, allowing GMs to prepare deadly ambushes where additional foes can spring forth from the sand itself.
Dune Collapse Sand dunes form in a "wave" formation, with one gently sloping side which the wind blows over, and one steep side which is blocked from the wind. Dunes can grow quite large, and oftentimes, quite precariously balanced. A sudden change of wind can potentially cause a dune to collapse towards the steep side. This would have similar effects to an avalanche (Pathfinder Core Rulebook Page 429)
Underfoot As noted in the Pathfinder Core Rulebook, hot deserts are not always endless wastelands of nothing but sand. Stones, sometimes a lot of them, often litter the landscape of a desert. Occasionally, even extremely resilient shrubs can manage to survive in the harsh desert landscape. So light undergrowth, light rubble, and dense rubble are all possibilities in a desert environment. See the Pathfinder Core Rulebook page 430 for more information on that.
Bones Perhaps it was once a large native creature, such as a blue dragon. Or maybe it was a pack animal brought here by travelers, or simply a wild creature who became lost. It could even be the remains of a creature who died here when this desert was lush and green. Wherever it came from, and whatever the cause of death, all that remain now are bones stripped clean of any meat by the persistent winds. While small creature's bones may only be good for improvised weapons, the skeletal remains of a blue dragon or dinosaur could potentially provide a good vantage point for archers, or just an interesting environment in which to fight. Hopefully the players don't try to use it as a landmark, however, since the nature of a shifting sea of sand is that it will sometimes completely conceal the presence of these bones under a dune of sand.
Big rocks are, without a doubt, the coolest thing about deserts. The unique element of sand constantly being blown around seems to create an environment where literally any kind of rock formation imaginable can theoretically be produced. The sheer level of variety makes it impossible to discuss in any meaningful way. Instead, here are a number of photographs of interesting desert rock formations. Let your imagination run wild here. Click any of the thumbnails for larger versions.
Not to sound like an advertisement, but if you like the options which a hot desert environment provides, and would like to run a session or even a campaign in a desert environment, you might consider picking up Sandstorm. It's one of the better Dungeons and Dragons 3.5 supplement, and it manages to retain most of its relevance in the Pathfinder system. The only reason it was cut from my recent list is because I thought it would be odd to mention it without the other "Environment Books" (Frostburn, Cityscape, etc), which I don't yet own and have not read. The book contains a lot of great information on running sessions in a dry desert, as well as some really cool desert monsters and specific locations to adventure in. I've wanted to find a use for the "Dry Lich" monster for ages. Now that I've got myself all hyped up to give deserts a try, I just might get a chance...