A lot of people talk trash about weak, common monsters, like goblins and rats but also especially skeletons. But the skeleton has a certain beauty.
The skeleton is weak but still fantastic; it sets the mood of the game early without overwhelming 1st level characters.
The skeleton is mindless; it cannot be negotiated with, which makes it simple for the beginner who may not yet grasp the finer subtleties of picking your fights carefully.
The skeleton is single-minded; it does what it is told and nothing else, so one simple trick is to include a room in a dungeon with a large number of skeletons ordered to attack only under a specific condition -- frightening for the nervous players of low-level characters, and yet easily avoidable by the clever player.
The skeleton is fearless; if you are diligent in making morale rolls for goblins and animals, the fact that skeletons attack without question makes them stand out.
The skeleton is horrific; most systems include some special rule, like minimal damage from impaling weapons, to reflect the fact that it can't be wounded in the same way a human can.
The skeleton is inhuman; killing hordes of them doesn't raise any morale issues.
The skeleton is lifeless; if monsters locked in rooms without access to food and water bothers you, the skeleton is always a viable alternative.
The skeleton is undead; it's an easy way to illustrate the difference between clerics and magic-users.
The skeleton is gruesome; there are so many opportunities to add disturbing details you might not be comfortable using with other creatures, such as hands that continue to crawl after being severed, or skulls being knocked off the skeleton and shattered against a wall -- and yet it keeps attacking.
The skeleton is modifiable; endless "varieties" of troll or spider may seem forced and a bit of a cheat, but the skeleton is, quite obviously, a chassis, practically a machine that cries out for additions: what is a lich, anyways, except a skeleton that's had a wizard's powers and personalities bolted on? The uncanny origin of the skeleton makes it easy to personalize, adding features and behaviors that fit with its specific cursed origin.
So join with me: embrace the skeleton.