After much thought, this is what I came up with (comments, suggestions & critiques are invited):
Size: Between 4’ and 6’ (aprox.)
Colouration: Beige to dark brown, with wide hair variation between black, brown, blonde, and red.
Lifespan: 50-80 years
Civility: Civilized; very common
Range: Humans live in almost any clime, shunning only the deepest forests, broadest deserts, and highest mountains. They rarely live underground.
Physical Description: Humans need little description. A common sight nearly everywhere civilization is found, their distinctive noses (larger than all but the Dwarf’s, and less rounded than theirs) and odour (undetectable by the species) are well-known.
The Human body is capable of a wider degree of variation than most other races. With the exception of the Elf, no other species varies so widely in height. Certainly no species has as many different types of build as the Human. The Human frame is well-designed for an active life involving both graceful, energetic movements and long, punishing marathons. They may not be able to work as hard or as long as a Dwarf or a Centaur, but they're able to accomplish a wider variety of tasks. The Human body is equally at home hauling stones and climbing trees.
This combination of strength and agility makes the Human more suited for warfare than almost any species. Those generals who have fielded masses of muscle against Humans have soon discovered how easily they can be out-manoevered and out-matched by the Humans' superior mobility; against troops of lighter races, the Humans can easily cut a swath through all but the most seasoned troops. The only species in civilization quite so suited for the battlefield as the Human is the Centaur, but they have neither the numbers nor the mindset to raise an army.
Psychology: The Human mind is determined, defiant, and often quite resilient. As befits a young race, they're open-minded and innovative, preferring to generate new concepts and ideas rather than simply embrace old ones. Once an innovation is obtained, it spreads rapidly throughout the species, who seem more likely to adopt a good idea when they see one than most others. The more sedate races, such as the Elves and Dwarves, see the Humans as impatient, flighty, and distractible.
The race has a talent for being talented. Unlike the Elves, who tend towards art and philosophy, or the Dwarves, who tend towards architecture and science, the Humans once again strike a balance between the two, having a number of leaders in very diverse fields. Humans are excellent planners and scientists, making fascinating discoveries in the fields of mathematics, geometry, navigation, and animal husbandry. They have an affinity for the sea and water, and are among civilization's most accomplished boatwrights. They make for the most intimidating warriors, and have many fine writers, poets and philosophers.
Society: Humans invented the city and haven’t finished their love affair with it yet. They group together in cities, nation-states, or empires, and fight fiercely amongst themselves for them. They are moderately xenophobic and require periodic scapegoats or circuses to vent the natural pressures of living in large groups. Gossip and news are also important.
Family is of import, but not so much as in some other societies; most Humans can only recite their family tree back three generations at most. Their attachment to family, though, is as strong as any others', even if that family is not related by blood, but by emotional bonds such as friendship. More than any other species, they are fanatically loyal to their crown or their flag.
Clothing: Almost always clothed, Humans are known to wear the usual civilized variety, although their penchant for heavy metallic armour of the most protective types is considered an oddity by other species. No other creature would come up with the idea of full plate mail.