Kael'thas Sunstrider, who was on her team the last time they'd seen each other, was in Outland. Not only was he in Outland, he was working with Lady Vashj to free Illidan from Maiev's grasp again. Hooray betrayal! Maiev, without most of her Watchers and utterly cut off from all of her resources (and certainly not in any position to receive reinforcements from anybody) lost Illidan once more. At the end of the battle, Maiev's fate was completely unknown. She was widely accepted to be dead, since nobody had seen her since she went through the Dark Portal, and it was painfully obvious she hadn't achieved her goals.
Enter The Burning Crusade. Illidan had been buddying up with the Burning Legion again, until he totally screwed up and failed Kil'jaeden. To protect himself, he holed up in the Black Temple with his quickly-fading sanity, enslaved most of the remaining native races of Draenor, and his allies were making a pretty crazy mess of things, with the whole... Kael'thas sabotaging the Draenei and kidnapping a Naaru, Lady Vashj seeking to drain Draenor of its primary water supplies and pissing on Neptulon, and all of that sort of thing. Pretty much proving why Maiev was right, and Tyrande was friggin' nuts. The Horde and Alliance took to Outland to try and get the place under control again before the Legion managed to do it first and set up a nice little homeworld for themselves right next door.
That distrust comes through in full force when we, the adventurers, finally enable Akama to follow through on his plan. Maiev is released, and while she sticks by Akama's side briefly... she takes the first opportunity that presents itself to break off and try to get the job done on her own. Now that she's had a taste of freedom, the last thing she needs (and probably the first thing she expects) is another betrayal by a so-called 'ally'.
When she finally reappears after splitting off from the other heroes, it's during the encounter with Illidan himself. If you weren't sure of her motivation before, it's made very clear during that fight. I'll let her heroic-arrival quotes do the talking here:
Maiev: "Their fury pales before mine, Illidan. We have some unsettled business between us."
Illidan: "Maiev... How is it even possible?"
Maiev: "Ah, my long hunt is finally over. Today, Justice will be done!"
The battle commences, and Maiev continues as an avatar of Vengeance: "This is for Naisha!" she yells.
When the battle is through, Maiev stands proud and victorious. That is, until a grim realization settles in.
Maiev: "He's right. I feel nothing. I... am nothing. Farewell, champions."
You can jump up and down and call her emo or spam /wrists at her all you want, but consider her situation right now. Really think about it. She has nowhere to go since the Night Elves totally screwed her over. She can't go back to being a Priestess, and her faith in Elune is most certainly shaken. The Goddess picked Tyrande over her, and Tyrande is a cold-blooded murderer. All of her friends and family are, as far as we know, dead. The job she's had for over 10,000 years is gone. She has no place, no purpose. What can she do with the rest of her life? Her only option is to start over from scratch, and she certainly can't do it with the Night Elves. Tyrande and Malfurion's actions burned those bridges.
She's a woman who, quite literally, had everything she has ever done, known and loved ripped away, and has been left with nothing. The only thing that kept her waking up every day is the knowledge that she needs to set things right in the world and deal with Illidan. Now that that's done... what is there for her? What can keep her putting one foot in front of the other now? We don't know, and neither does she.
That's why she's my favorite character. She's not a hero in the regular sense of the word. She's not a villain, either. There was no 'right' or 'wrong' when it came to Maiev's actions in the greater sense of the story. She was a person driven by what she felt was right, and even when the entire world was against her, she clawed her way toward her goal little by little.
And that's why I want to see her make a comeback one day in Warcraft. Did she just lay down and die after the encounter with Illidan, contented with the fact that she achieved the vengeance she's been chasing for so long? Will she take up a role somewhere else in the world? What will she think of everything that's happened in Azeroth since she went through the Dark Portal? Would she buddy up with some other faction in the Alliance? Will she avoid the Night Elves entirely and set up shop as a neutral entity?
Maiev has became a character where almost anything is possible, and that is a pretty exciting idea.