It is unfortunate that "death" in comic books has become such a cliche and it is unfortunate that it started because of this comic. When this comic book came out, the death of a hero in comics was a rare thing.
Jean Grey was a founding member of the X-Men. While her early days were less than stellar (Snell has done a good job of finding some ludicrous use of her power HERE on his blog "Slay, Monstrobot of the Deep!"), she took a major turn and became one of the "big guns" in X-Men #100, shortly after the "new" X-Men arrived on the scene.
Her story played out over the next 37 issues, culminating here. If you know comics, you know her story. If you don't, all you need to know is absolute power corrupts absolutely and the power Jean Grey was given in issue 100 was absolute.
Before the retcons and resurrections, the all too human Jean Grey could not control the power within her. She made mistakes and her actions caused the death of 5 billion sentient beings. She was sentenced to death.
In this issue, the X-Men fight to save her....to prove that she can be reformed and control the power.
The X-Men are wrong.
As Jean Grey fights herself to remain human and control the power that rages within her she realizes that there is only one way to save herself, her friends, and possibly the entire galaxy.
Cyclops' wrap-up is talky by today's standards, but to me it is still powerful. And like the "best" death scenes (see the runners-up below) it has been rendered moot...