No, the Horde has Not been "extremely aggressive." Had they been, the Alliance would probably be extinguished from the planet.

Anyway, Wrynn isn't extra-impulsive or emo or whatever. He's just stupid.

Thrall has a much, MUCH harder job than Wrynn. The Alliance are extremely easy to control, just ask Onyxia. Let's see what Thrall has to do...
1) Reign in the long-held traditions of extreme violence and
barbarism of the trolls
2) Somehow marshal the Forsaken's energies towards fighting for what is right, which he Succeeds In Doing despite the fact that they are Undead and want to kill everyone; the fact that they kill a few living people now and then for experiments only means they aren't waging war on the Alliance and inflicting
incredibly massive casualties, which they would do without Thrall (and may someday do, but until then Thrall's the only barrier between them)
3) Forget the fact that he has had to watch ALL of his people suffer for years in internment camps
4) Rule the orcs, who have only known war and bloodlust, thinking of Azerothians as their enemies, for dozens of years
5) Get the whole Horde to fight for the greater good and put aside their differences with the Alliance, which to a large part he does, somehow putting in an equal share of the burden of saving the entire world--that isn't yet truly their own

Thrall has been so successful, it's freaking AMAZING, and when he fails at all, such as when he's betrayed by the Forsaken, he takes it personally, despite no obligation to be the kind of diplomat that he is.

If Varian wasn't Stupid, he would not expect the Horde to be like an ancient alliance of civilized races, would see how little the New Horde has actually attacked the Alliance since Thrall started leading it, and see that Thrall tries to quell the manifestation of the New Horde's old feelings--Garrosh.
Varian wants open war with the New Horde, costing countless lives, rather than letting Thrall continue to try to get the Horde into a new shape, which takes time and will cost very, very few lives comparatively.

It's hard to believe the number of posts here of people that want to believe that you can compare the Horde as a government with the Alliance. You can't assume that Thrall has the same sort of tight power over his people as Wrynn, because the Horde is quite simply a horde. And if you think that this lack of initial discipline--read: that can only be built over time or else you get rebellion--means the Horde should be eliminated, like Wrynn does...well, you should think about it some more.

So Varian's just very shortsighted, very arrogant, and very stupid. Probably because of his experiences.