"I heard if you cut off an extremity, it'll regenerate a little bigger..."
First, a note about troll biology: As you know, trolls are famous for their tusks, big ears, and for their tall and lithe look -- males stand at about 7 feet tall when they're not hunching over. You also may have heard that they have the ability to recover from any injury, and even regenerate severed appendages! What you may not have known is that troll skin is not necessarily what it appears to be at first.
Trolls are divided up into different groups depending on which environment they have adapted to: Ice, sand, forest and jungle. Ice trolls have white skin that almost blends in with the snow, while sand trolls have scorched, dry skin, beaten down by the harsh desert environment. So far no surprise there -- but forest trolls possess a strange mutation that makes moss grow on their skin, thus resulting in their odd green color. As for the jungle trolls, which is the group all Darkspear trolls originally came from, their bodies are covered in a short, soft fur which gives them their blueish or purplish coloration. Remember that the next time you hug a troll (and survive) -- not only is he tall and deadly, he's also soft and cuddly too!
"The way to a man's heart be through his stomach, but I go through the rib cage."
And now we turn to trollish history -- and here we get something of a reprieve from the convoluted path that the orcs have taken, as the history of the trollish brethren in the Horde is much simpler. This is notwithstanding that trolls may very well be the oldest race on Azeroth, with a history going back 16,000 years, and cultures which predate most any other in the world. This history doesn't strike me as something that every troll would know about however, as they seem to be much more of the type to live in the present rather than dwell on the past; I imagine many trolls would be surprised to hear about how their race once fought off an unstoppable onslaught of hideous giant insects, and may have even been the progenitors of the night elves. If your troll deems himself a scholar, however, by all means dig up the rich history and see what you discover.
One thing has never changed about trollish history: warfare. Perhaps bolstered by their ability to recover from any injury short of death, the trolls remain the most savage and bloodthirsty of all the Azerothian races. Their entire history leads from one war to another, almost unceasingly -- whether they were fighting insects, or elves, humans or even one another, the trolls have always been fighting something, and much of their religious superstitions are built up around the prosecution of killing, and protection from the spirits of the dead.
"I got a shrunken head -- I just came out of the pool!"
As a member of the Darkspear tribe, you would have enjoyed uncharacteristic (as far as trolls are concerned) peace growing up under the leadership of the wise and spiritual Sen'jin. Having been driven away from your ancestral home in the jungles of Stranglethorn Vale centuries earlier by the other bullying troll tribes, the Darkspears gradually bore all the shame of defeat and exile and made a home for themselves on a rain-swept island in the middle of the ocean. Eventually, however, some humans from Kul Tiras showed up and made a settlement there as well, and your murloc neighbors started showing more and more signs of violence. Your leader began to worry about the future of the Darkspears until, a few years prior to the current setting of World of Warcraft, he received a vision of an orcish seer who would save his people and take them off the island.
Sure enough, Thrall and his orcs sailed on by just as the conflict with the murlocs was about to reach its climax. His ships were ravaged by the Maelstrom, and he needed time and assistance in repairing them. Together, the orcs and the trolls drove the humans away from the island, only to be captured by the murlocs soon after. Sen'jin was unfortunately sacrificed to the Sea Witch that had been inciting the murlocs to violence, but just before the troll leader died, Thrall managed to escape and hear Sen'jin's dying words: a plea to save his people and take them from this island.
Your troll character might have gone with Thrall to fight in the Third War alongside the orcs, or she might have stayed behind to weather the wrath of the Sea Witch with Sen'jin's son Vol'jin as the new leader, until a year later, when the whole tribe could finally make the move over to Durotar, where the orcs had founded their new nation. Vol'jin and the Darkspears set themselves up on the Echo Isles in the southeast of Durotar just before another wave of humans showed up under General Proudmoore showed up and started causing all kinds of new problems for the new Horde. Along with the hero Rexxar, the tauren and some ogres, they helped the orcs fight off the humans.
But once again, the victory was short-lived. One of the Darkspears' witch-doctors, named Zalazane, started mind-controlling people and taking over the Isles for himself! Vol'jin retreated to the mainland and founded Sen'jin Village as an outpost from which to fight back against the traitor and his mind-slaves.
"I got all this, and personality too!"
In spite of their culture having been based on a twisted misunderstanding of voodoo and primal societies, and notwithstanding some troll's apparent reluctance to give up the bloodthirsty, cannibalistic, head-shrinking ways of the past, there's something about the trolls that appeals to a lot of people. Something about them strikes you as care-free in the face of constant difficulty, strangely cheery, even when talking about serious topics like their tribe's close brush with extermination, or the return of the ancient Blood God, Hakkar.
Whether your character is a truly enlightened member of the new Horde, or an utterly cruel savage underneath that jovial attitude, he or she faces the same challenge as all the other races in World of Warcraft: to adapt and grow in the face of great changes in the new world. There's something about the troll that proclaims how they'll grow and change in their own unique way.
For further reading about trolls, check out the massive Troll Compendium from Blizzard's official lore site, or, if you prefer, WoWWiki's page on the same topic. Be sure to read up on trollish voodoo for a clear sense of where your tribe is coming from culturally, even if your character is only marginally interested in faith and religion, and especially if you want to be a priest, shaman or some other sort of spiritual leader. Also have a look at the simplified Dramatis-Personae page for more suggestions on creating a troll (but be aware that Dramatis-Personae seems to have missed the part of the story that has the Darkspear trolls living on an island for the last few centuries, rather than in Stranglethorn Vale -- other than that, their take on trolls as essentially evil is certainly a valid option).