I leveled through the new troll starting area on the beta and that's something I thought about while writing this article, but Vol'jin's "conversation" with Garrosh isn't really equivalent to Varian's premature threat. This is how I see it, although I acknowledge that not everyone will:

The conversation between Varian and Thrall is a meeting of ostensibly neutral parties: The Alliance and the Horde are not in a formal state of war at the time, and still -- strictly speaking -- aren't now. However, Varian's threats at both the Undercity and in Dalaran make it fairly obvious that the Horde should prepare for Alliance hostilities in short order. I like Varian as a character a bit more than I did previously (though I still think that Blizzard made a serious mistake leaving so much of his characterization to the novels and comics), but by any measure, this is still dumb. His threats aren't being made the weight of other Alliance leaders brought to bear; they're emotional responses to stressful situations.

Vol'jin and Garrosh are allies when their conversation takes place, and still allied afterwards. Vol'jin's been around the orcs long enough to know that their politics are conducted with a sort of chest-thumping bravado; there's a certain peacockesque quality to how the orcs run things. They strut and puff and pontificate about honor and a host of other inconsequential matters while attempting to deal with something. He also sees Garrosh eject all non-orc/tauren from outside the center of Orgrimmar because they're "too weak" to adequately defend the heart of the city. Vol'jin isn't declaring war on the orcs; he's attempting to influence Horde policy. His threat is being made from within the context of Horde politics, not as a challenge to the system as a whole. I see it arising from the need to convey both trollish strength and the stupidity of pointless bellicosity to Garrosh. The ultimate aim is to remind Garrosh that not everyone is on board with what he plans to do.

Varian and Vol'jin's threats look similar on the outside, but they weren't meant to accomplish the same ends. Varian's outburst is an early, unofficial declaration of war; Vol'jin's threat is a defense of his race and a reminder to Garrosh that he can't make policy without deriving consent from other Horde races. The Darkspear don't intend to leave the Horde; they need the orcs. Unfortunately for Garrosh, the orcs also need the rest of the Horde races, particularly if Garrosh can't keep his dumb mouth shut.

Anyway, that's how I see it.