Me and my friend recently got into a bit of a lore discussion, and I figured it might be worth bringing here as well. It's in my opinion that the biggest moment in Wotlk was the Wrathgate.
Let me defend the position here.
((Spoiler warning for any slow pokes out there))
The reason I think this is because of all that the Wrathgate did, and how far it stretched. The actual events of the Wrathgate brought us this;
First of all, Saurfang the Younger died. He showed the utmost bravery, and it wasn't enough. Another thing, he and Bolvar showed that the Alliance and Horde were willing to work together, and able to. There was no backstabbing from the Orcs or Humans. Another scenario that transpired was that the Forsaken, after many long years, had finally perfected their blight. It was a huge moment for them.
Lets keep in mind that this is all established in the first minute or so of the cinematic. Now, what happened next? Betrayal from within the Forsaken. Putress orders the Blight to be used not only on the Lich King, but the fighters down in the field as well, killing many Horde and Alliance, and leaving Bolvar to die, were it not for the Red Dragons.
From here on, this is the ramifications of what this event brought.
This all led to a full scale invasion, on both sides, into the Undercity, but the biggest impact was from the Alliance side. No one lore wise had been in Lordaeron's capital city since it fell, for the Humans. To see the city disgraced like that, to see how the sewers were, all the plague, all the death... It'd be like leaving your home, only to come back to it being a funeral home. Things were already bad for you, believing that the Forsaken had slaughtered the Human's greatest hero at the time, Bolvar Fordragon. as they progressed, they eventually came head to head with Thrall, and tensions erupted. The war was back on. The peace that Thrall had worked so hard to achieve was shattered. Sylvanas was banned from using her new blight, and put under constant watch by the Kor'kron. Saurfang JR was eventually raised as a Death Knight, and Bolvar became the Jailor of the Damned.
It's only recently that these events became settled. It took 3-5 years, lore wise, for these events to be even remotely settled, the war having been the biggest impact of it.
That's why I think the Wrathgate was the biggest event in WOTLK, and perhaps WoW history, in terms of Horde vs Alliance, and second only to the final events of the Cataclysm. What do you guys think?