Of all the commonly appearing fantasy races, trolls seem to be the most varied and divergent - even within their own realities. I endeavored to record the features of the 'basic' troll present in each.
In case you were wondering (and how could you not?) This series of sketches is being recorded in a 3.5" x 5.5" Moleskine Pocket Sketchbook, using a Pilot Better extra fine retractable ballpoint pen (black ink). There are no preliminary sketches or pencil roughs - good or bad, everything is presented as it was put down in a single pass.
Dungeons & Dragons (Monster Manual I, 4th ed.)
The face of a troll is dominated by its long, pointed nose. The mouth is wide and filled with relatively small - albeit sharp - teeth. Proportionately small, the pointed ears sweep back and out, and have a tendency to droop. An unkempt mane of greasy black hair is not uncommon in the species, though it does not appear to grow anywhere else on the body in any significant amount. The spine connects to the skull further back than in the more civilized races - a trait more commonly seen in lower order carnivores. Consequently, the overall posture takes on a slouched appearance, with the head thrust forward.
World of Warcraft
Like the goblins of the Warcraft world, trolls have large ears and long pointed noses. The former being more swept back and the later being thinner and only slightly more in proportion to the face than their smaller Horde-mates. Additionally, their facial structure tends to be longer and generally more narrow, in proportion. However, it would appear that the defining feature among trolls in this reality are the two large tusks that grow down and curve up out of their mouths. Hair may be present on the face and head, and tends to be a warm, bright color of a contrasting nature to that of their cool-hued skin.
Warhamer Fantasy Battle (7th ed.)
Most animalistic of the specimens observed, the Warhammer troll has thick, mottled skin bordering on the verge of being reptilian in nature. Indeed, the regenerative properties of the troll are not unlike those observed among reptiles and amphibians - though that topic extends beyond a casual taxonomic discussion such as this. The mouth is large and filled with oversized, misshapen teeth that jut out. The nose is large and flat with small nostrils, and the ears are little more than ragged scraps of flesh that sweep back and sag downward. The rearward connection of the skull and spine that is present in the D&D troll is even more pronounced in this specimen, exacerbating the hunched appearance.