Jean's voiceover + the phoenix shape in the water.
The events leading to Jean's death served one purpose: To kill her. And the only reason to kill her was to set up the sequel. While they hinted her death from the beginning, the actual event was handled badly. It's rashed and manipulating. It didn't push the plots (of X2) along at all.
What's worse is that 3 minutes after her death, we're told our tears were wasted. "Haha, gotcha!" It trivialized the death and the impact on others. What could be noble and heart-wrenching became nothing more than a plot device to squeeze some tears.
I disagree that the audience should know Jean's "rebirth" in advance. A grief-stricken Scott will be interesting. The dynamic among X-Men team members will be intriguing now that Jean's gone. The return of Jean will be more shocking. And the reactions of this news, the efforts put to bring Jean back to the fold more dramatic. Now, we can still have all of these in X3. But I'm afraid that I won't enjoy it as much because I'll be constantly reminded that their pain, sacrifices, efforts... are for nothing. (She's alive, you silly!)
