Playing A Troll by Marchais:
I started typing up some long-winded thing, but decided keeping it short (like the format Quill previously used, which I found quite effective) was a better idea to keep from coming across as some kind of strict rulebook that roleplayers should never defy. I can't say I have the same reputation as Quill, but hopefully this will prove helpful anyway. I'll add to this anything anyone else contributes that looks well-thought out(not necessarily that I agree with, and I will do my best to be fair.)
You were not born on the Echo Isles. You might have been born on that island in the South Seas, or in Stranglethorn. I honestly cannot find a date as to when the Darkspear were kicked out of Stranglethorn Vale. However, I know for certain that they only moved to the Echo Isles within the past couple of years, meaning any Troll born there would be a young child, not an adventurer or soldier.
You do not need an accent. I know a lot of people stumble over this one, it's hard to type believably in an accent you don't actually have. Actually, it's hard to type in an accent period. But there is no actual need for you to have one. If you don't feel like you can do the accent well, then just leave it out. No accent is better than a bad accent that no one can understand.
Playing other tribes. This one is tricky because while I believe it is perfectly reasonable, I know there are plenty of people who don't. For some tribes or species, it would be easier than others(a Forest Troll could fit into the Horde much better than, say, an Ice Troll.) But, before you do decide to play a non-Darkspear, you should always look up some information on your intended tribe. Try and figure out how they would fit into the Horde, what their relations would likely be with other races, particularly the Darkspear. A Forest Troll, for example, could be seen by Darkspear with maybe a bit of rivalry; their two empires never really got into serious warfare when they were in control, and after their respective falls they didn't have anything to do with each other at all. At the least, another Troll is better than something else. To an Orc, a Forest Troll could be viewed as either an acceptable comrade(they were once allies, and the Revantusk Forest Trolls are still allies) or as a cowardly traitor because of Zul'jin's refusal to rejoin the Horde. To a Blood Elf, of course, you'd be hated.
The gods are selfish and everywhere. The Troll concept of religion seems founded on the ideas that spirits have their own goals and aims and are regularly involved in the daily lives of mortals. Though many of the old practices have been forced underground in the presence of the Horde, it's difficult for a people to suddenly ignore what was once a key aspect of all their lives(especially in the context of Warcraft, where spirits can and do have tangible effects on the world.) Few Trolls worship their gods because of a sense of higher duty; they believe their gods and spirits are imperfect and fallible, but are a hell of a lot stronger than they are and, hopefully, stronger than their enemies. Sacrifices are made and rituals performed to make sure the spirits lend their aid to your people, and not someone else's.
War isn't civilized. Their view on gods and spirits probably transfers pretty well to their view on warfare; look out for number one. War is about winning, killing the enemy so the enemy doesn't kill your people. Almost every Troll, regardless of tribe, has seen their people constantly besieged by enemies from all directions, due in at least some part to their people's own hatred of others. Duty and honor aren't concepts completely unknown to the Trolls, but in an environment where even your own kind are potential enemies, there's little room to foster such notions. Trolls don't really seek glory through combat the way Orcs do. To them, war has been a means to an end, be that security or new territory, and for the last several thousand years Trolls have just been trying to win back the lands they used to hold.
You don't have to hate Blood Elves. Although it is very likely that you do. Blood Elves certainly won't appreciate you very much, given how similar you are to their hated enemies, the Forest Trolls. That, and in addition Trolls seem to be naturally adverse to things that are really different, and the Blood Elves are completely different. It will mostly come down to individual personality.