We know that Kil'jaedan is supposed to be half-summoned through the Sunwell, and these animations seem to hold that out. he appears only from the waist up, so we can assume he's otherwise submerged in the Sunwell, and probably trying to crank out a last bit of energy, possibly by siphoning it from Anveena, to completely cross over. Luckily, 25 intrepid heroes show up at the last minute to stop him. Now, what is most intriguing to me is that he doesn't really appear to definitely die. Instead, his "defeat" animation suggests that he is simply sucked back into the Sunwell, apparently returning from whence he came. It is certainly possible that the Sunwell simply destroys him, but the way he sort of slides back in would suggest to me that he's being pulled back through the portal. His scrabbling at the sides of the portal, then snatching at his amulet, might support that reading. He's not in the throes of death as much as he's trying to keep his hold in this world.
Now, honestly, I'm not against the killing of previous lore figures such as Illidan and Kael'thas. I believe that every character comes to their end sooner or later, and our characters have done enough in the world to deserve a chance to actually emerge victorious over enemies such as them. That said, I know quite a few people have expressed dismay over killing off characters like this, so perhaps this is Blizzard's nod to them, allowing Kil'jaedan to be defeated without actually dying. Of course, the Burning Legion is pretty much the biggest of the big bads in Warcraft lore, so it may be that Blizzard wants to save Kil'jaedan for a final showdown with Sargeras somewhere down the line, or something to that effect. Either way, I'd say we can't count Kil'jaedan out yet.
Another interesting thing to note is that his amulet apparently flies off, and while he desperately grabs at it, it seems to stay in Azeroth while the rest of him is dragged down into the portal. This, of course, leads one to wonder if the amulet itself has any importance. Is it simply a really uber necklace with lots of spelldamage, or is it something more? Kronack wonders if it might be related to the Demon Soul. That would seem unlikely to me, since last we knew of it, it had been destroyed, and it was a toy of Deathwing's rather than of the Burning Legion's. Still, it could be something that operates under a similar principle. Perhaps it will show up on Kil'jaedan's loot table and we'll learn more about it then. It would be pretty cool if the Demon Soul wasn't as destroyed as we thought, and Deathwing lent it to Kil'jaedan, but a plot twist like that would be a longshot, I think.
What do you think? Am I reading too much into a few boss animations, or do you have theories of your own?