Elementals do not die, they are banished to their elemental plane. What remains is the shell of their material embodiment. Killing them on their home plane (where they, naturally, are much stronger) will usually finish them off for good.
As for rehashs, I can't understand peoples hatred. I mean, it's not like it was Naxx, only thing copied over was the terrain. You get to go to place you did something in years ago, and see how the place turned out after that. The content is new, and the story gets told on. In many ways, they are actually BETTER than some completely new 5-man.