Atmosphere. The seedy bar with dim lighting, mismatched/broken/mended furniture, smoky air and grimy unwashed patrons gambling coin in the corner. The dank, dark, dripping cave with a small stream trickling through it, bats in the ceilings and centipedes as long as your arm creeping around limestone stalagmites/stalactites. The tall leafy forest with thick jungle undergrowth, dim beams of light shining down through the canopy and vines and poisonous spiders/snakes on every other branch. The alleyway, narrow and garbage filled, with hard packed earth on the ground, alley cats/dogs/rats and sewer grates. The merchant stall/shop with an assortment of wares to choose from (insert your required weapons, armour, clothing, food, basket weaver here).
That is: Describe a scene with a suitable atmosphere, include various descriptions of plot objects and misc objects. Include visual (smoky room etc.), audio (conversation, clink of coin/mugs), olfactory (the smell of smoke/tobacco/unwashed bodies) and tactile (the sticky floor from beer spills) senses. It ought to include obvious things (people, potential dangers, entry exit points) that characters need to know straight away, as well as secondary things which can include other 'plot object/s' and 'red herrings'. Possibly followed up with a second description that highlights the plot object/s and dismisses the red herrings in the event that players miss or latch onto the wrong things.
As another thing that is possibly useful for GM writing, is a list of suitably descriptive words that will evoke an adventurous atmosphere. Instead of creeps, why not slinks? Instead of tall, why not soaring? Instead of dark what about inky or sinister? If you want to encourage GM creative writing, having a list of adjectives, verbs and adverbs to inspire as a starting point might not be a bad thing, as well as some examples taken from literature or created by other MW GMs so people can see what sort of stuff is good descriptive writing. I learn best by seeing and imitating.