The death of a major character was still pretty rare in 1980, and the only one that really compare was Gwen Stacy.  Gwen's death (along with Norman Osborn's) was probably the most shocking and controversial.  But it, along with Charles Xavier's "fake" death earlier, set the tone for character resurrections.

Jean's death was one of the first of a major active hero (Charles Xavier was the first, but the Swordsman was really a B-list villain turned hero and Thunderbird wasn't really that established) and the biggest death until they decided to kill of Mar-Vell a couple of years later (which thankfully has stuck).

When Elektra was brought back less than a year after her death I think the idea of deaths being permanent was pretty much out the window, and now what major character hasn't been killed and resurrected at some point?

From my perspective, I started collecting Marvel comics with a vengeance in late 85, at which point Marvel was both in the process of bringing her back and giving birth to a different Phoenix, so I never felt a direct impact from Jean dying.

The story (I hesitate to say 'event' because it has different connotations) certainly influenced what came after. Killing Jean was obviously an organic and unplanned affair, though it seems the impact unfortunately spurred creators later on down the road into offing characters as a cynical ploy to attract attention to a title. I'm thinking of A Death in the Family, the Death of Superman and so on.

I'm curious as to whether DC would have got rid of Supergirl in quite the same manner they did in Crisis if it was not for the Death of Phoenix half a decade earlier.

That I did!

That was a Life Model Decoy, so Talbot has remained dead.

He must not have been paying attention to the Gerber/Marvel Howard the Duck situation.

Robert, you omitted a few early Marvel superhero deaths:

Bucky Barnes (1963, though the event was retconned to have happened in 1944)

The original Human Torch (1966)

Toro (1969)

I believe all of the above have been resurrected at some point post-1985, right?

and in terms of deaths of supporting characters, there also was:

Ben Parker (1962)

Franklin Storm (1964)

These latter two would fit into the category of "created to be killed" rather than the killing off of established characters.

I agree that Jean's death did open the floodgates of character deaths... for the obvious reason that fans were becoming the dominant share of the market, and X-Men #137 demonstrated that fans would buy up multiple copies of any issue with a major character death.

edit - They also did the same with Red Raven (single appearance), Marvel Boy (single appearance) and Miss America (flashback)