TL;dr warning!!!
I just finished watching TTGL, and for the first 26.5 episodes, I was ready to proclaim it as the best anime ever. At the 20 min mark of this episode, I was very motivated, but at they end I felt rather shattered. During Hotel Rwanda, Turtles can Fly, (IRL based fictional movies), and all the supposedly sad films the 2000's had to offer, I sad completely poker faced through. But seeing the weeding scene in TTGL, and hearing Love Conservation made me muster all me strength to not break down.
I love the fact that you people feel as passionate about the ending as I do. I've read through pretty much this entire thread over the past few days, and I can understand everyone opinions. Now I don't consider myself an expert at literature and the like, but I'm almost certain that Nia's death was a diablo ex machina.
Ignoring philosophical arguments and the theme of the show, the fact remains that Simon said he would return her back to normal, not that he would save her. And last time I checked being a normal human doesn't imply that you die when the alien race does. It would make more sense, if she was one of the bodies in the AS world, but she wasn't. She simply had been implanted with AS genes that were to trigger her transformation into an AS life form. If anything, destroying the AS should have helped to cure her of AS genes, as if you kill a puppet master, the puppet doesn't disappear, it stops being a puppet.
That said, her death was clearly a diablo ex machina, or in other words, a means to an end, with that end being a downer or bitter sweet ending to the series. This is strengthened by the fact that its Gainax's M.O. but seeing as the show probably became more well known than the company, I feel like they should have created a directors cut alternate ending. I've read both of Gainax's manga alt universes, and I've got to say, while of course not nearly as epic as the original, their endings were a lot more satisfying, what with the never ending simon x nia adventure.
So in the end, I love Gurren Lagann, I love Nia, and I hate Gainax.
TL; dr
prove me wrong. (jk)
P.S. after a few days of letting it sit, I don't feel that sad about her death and the ending, but rather betrayed by the way Gainax, ended it. I feel in my heart that she shouldn't have literally disappeared, and he shouldn't have figuratively disappeared. And in this way, that wouldn't of happened in the GL universe, or at least she would've been back with simon like J.C. and the apostles. (aka their still alive and well in my mind) I'm just now sad that I'll never see anymore of their adventures. Nia's irony is totally unique, whenever has a happy go lucky character turned out to be anti-social personality? I haven't really seen a character go from chirst figure to satan figure, to double christ figure?