I look first at sustained peak, which means more than just a season or two. And yes, it also means more than just parts of seasons (sorry Lindros and Forsberg).
I rate guys like Francis and Delvecchio lower than most because they were never gamebreaking talents.
But longevity is also important, at least while that player is maintaining a high level.
Examples:
For me, Brodeur's longevity at this point is enough to push him over Sawchuk in value, even though Sawchuk had the better peak.
Lidstrom might be slightly behind Potvin in peak value, but he's so far ahead in maintaining his standard of play, I rank him over Potvin without a second thought.
But Messier playing 6 more seasons and accumulating stats after he ceased being a star player adds nothing to his value.