...Varian's funeral.
Yes, Varian was technically at one time his gladiator slave. But honestly, Rehgar did quite a bit to mentor him as well. Rehgar easily saw the potential in Varian to be a great man, a great fighter, a great leader and did much to foster this potential and act as a mentor.
It was Rehgar who gave Varian his trademark blade, Shalamayne. Who taught him the story of Lo'gosh, who molded him into a fierce strategist and peerless combatant. Moreover, when it came time for Varian to leave, he just let him walk away without a fight. In fact, he knew it was going to happen well in advance, and did nothing. He knew that the man had a destiny to fulfill, and in this realization also found his own destiny.
So why is it that the orc who saved the fallen king in his previous time of need, who invested so much into him as a champion of the gladiatorial arena and helped him get back on his feet, and in return found his true calling as a champion of the Earthen Ring rather than a slave master as a result, is not there to pay respects to one so pivotal in his life?
It's probably one of the biggest lore gaffs in WoW ever. They could even make an amazingly touching scene of it, and let Anduin play up to his unity card in the process. Maybe a throwback to the ICC scene where his father lets Saurfang collect the body of his son. It just feels like a missed opportunity for an amazingly touching moment and some much-needed character development for one of the most sadly underutilized orcs in the Blizzard universe.