While the Trolls gladly agreed to the alliance with the noble Tauren, the alliance with the Forsaken had the Darkspear shaking their heads and wondering what Thrall was thinking. By and large the trolls don't trust their Forsaken allies, possibly viewing them as angry spirits that could never be set to rest and therefore were destined to bring only misery and strife to those around them. They are tolerated, but barely. As for the blood elves ... it is not really known in game one way or another how the trolls feel about the blood elves, but there are several key things to consider when trying to decipher how the trolls feel about their pointy-eared "kin" -- trolls live a very long time, and their memory is even longer. History is well documented in story and song, and the history of the trolls and the night elves is well known, as well as the hatred that the two races hold for each other. The blood elves, while exiled by the night elves, are still elves, which isn't apt to make any troll particularly happy. Add to that the blood elves' addiction and practice of fel magic, and it is largely unlikely that the trolls care for the elves any more than they care for the Forsaken.
Meanwhile, Vol'jin remained at Thrall's side in Orgrimmar, steadfast as always. In Wrath of the Lich King, the trolls continue to help their Horde allies in Northrend, and when Sylvanas and her people are thrown out of Undercity, Vol'jin steps up to help take it back alongside Thrall, despite whatever reservations he may have about Sylvanas or her people. The trolls of the Darkspear tribe have gigantic amounts of respect for Thrall, and willingly put their lives on the line for the Warchief. When the most powerful witch doctor and leader of your people experiences a vision telling you to follow someone -- you follow him. To the Darkspear, Thrall represents the salvation promised to them and foretold by Sen'jin. Despite his odd tendencies of having some ... strange choices in allies, he is still the orc that Vol'jin follows with all his heart.
Also of note in Wrath are the trolls of Zul'Drak -- while the Darkspear haven't taken direct interest in the proceedings there, players can find the troll encampment of Zim'Torga, which is neutral to both Horde and Alliance. The Zandalari of Zim'Torga are particularly interested in the madness that has overtaken the Drakkari tribe of Zul'Drak, and seek to try and save some of the Drakkari history before it is wiped out for good. While the Darkspear don't have a particularly overwhelming presence in Wrath, they can still be found here and there, helping their Horde allies, and have taken up arms at the Argent Tournament in Icecrown to further train warriors for the battle against the Lich King.
Also in Stranglethorn are another group of jungle trolls -- the Zandalar tribe that once persuaded the tribes of the Gurubashi Empire that Hakkar the Soulflayer was up to no good. The Zandalar tribe made a home on Yojamba Isle in Stranglethorn to recruit help against Hakkar, who has been reborn inside the ruins of Zul'Gurub. The Zandalar tribe is neutral to both Alliance and Horde, and holds no apparent interest in joining the Horde, being largely concerned with Hakkar's return to the world and his plans to devour it whole. These trolls are of the same tribe as those up north in Zul'Drak.
There is also one other troll tribe currently "allied" with the Horde at present time, this one up north -- the Revantusk tribe of the Hinterlands. The Revantusk were once part of the northern Amani Empire, but found themselves nearly wiped out by the Troll War between the high elves, humans and trolls. While the Revantusk tribe decided after deliberation not to join the Horde, they did agree to a pact of friendship and mutual assistance between the two and thus allow Horde players into their village and offer quests. Oddly, while the Revantusk were present during the Troll War and more than likely saw a good portion of their people slaughtered by the high elves and humans, the Revantusk are not hostile to blood elf players, instead treating them as any other Horde member.
The same could not be said for the rest of the Amani Empire. The trolls of Zul'Aman, led by Zul'jin, were absolutely furious with Thrall's choice in new allies. Small wonder, given that the high elves put out one of his eyes and imprisoned him during the Second War, a war that he fought alongside Orgrim Doomhammer and the rest of the Horde. Zul'jin swore that he would kill everyone -- Alliance, Horde, it didn't matter really as both were just as bad as far as he was concerned. Players got to battle and defeat Zul'jin in Zul'Aman during the Burning Crusade expansion.
While the Revantusk and Shatterspear are by and large friendly, they haven't allied themselves with the Horde in the same manner as the Darkspear. But even though Vol'jin and his followers are fiercely loyal to the Horde, the memory of Zalazane's betrayal haunts them to this day, and Master Gadrin continues to work towards reclaiming the Echo Isles. The Darkspear may be members of the Horde, but they don't have a real capital of their own, and have spent the last five years of the game working with the Horde rather than tending to their own issues. With the defeat of the Lich King and spirits at an all time high, Vol'jin has apparently decided it is time to take the Echo Isles back by force.
And that's what Vol'jin really desires above all else: to provide the home for his people that his father foresaw so many years before. Zalazane's treachery was essentially spitting in the face of Vol'jin, of Sen'jin's dreams, and although retaking the Echo Isles means that the Horde will have another city to bolster their forces from a strategic standpoint, it's the restoring of his father's legacy, and honoring his father's spirit that drives him. Sen'jin was a beloved leader of the Darkspear and his death, his vision, and the efforts of Thrall to save him will not be forgotten.
The Darkspear tribe is perhaps the strongest ally that the orcs of the Horde have. Fiercely loyal to Thrall, the Darkspear have literally changed their everyday way of life in an effort to meet the orcs' approval, going so far as to swear off cannibalism and voodoo, two things that were an integral part of their lives prior to their introduction to the Horde. The orcs' shamanistic ways appealed to the trolls, and their concept of honor was similar to the same honor the Darkspear lived by. That, combined with the vision of Sen'jin, sealed the fate of the Darkspear with the Horde, and with Thrall. In addition, they are fast friends and hold a great deal of respect for the tauren, whose shamanistic ways also appeal to the Darkspear. While Thrall has chosen some strange allies, it is his spirit and his heart that speaks loudest to the Darkspear tribe, and they follow him whole-heartedly. The Darkspear trolls live by their word, and the oath that Vol'jin swore to Thrall is nigh unbreakable.
The trolls of the Darkspear tribe are at their heart one-hundred percent behind Thrall; noble creatures of honor and creatures of their word. With the upcoming changes in Cataclysm, the subject of the vow of loyalty they swore themselves to is suddenly going to be taking an extended leave of absence, and his replacement is someone with whom the Darkspear have had very little to do with. In his absence, will the trolls of the Darkspear continue their work with the Horde? Will Vol'jin continue his work as advisor, or will he simply remove himself to the Echo Isles to lead his people directly? How will the Darkspear react to the venerated subject of Sen'jin's vision, the orc that would supposedly lead them to salvation, up and leaving the Horde in the name of whatever it is that takes him away? The answer is as yet unknown, but we won't have to wait until Cataclysm's release to see the beginnings of the Darkspear's efforts -- the liberation of the Echo Isles will take place shortly before Cataclysm's launch.