It captures a particular romantic myth that is as irresistible as it is far-fetched, and there is a now a kind of retrospective convergence that elides the clunkier parts of the story. What happens to the characters is something that now reads better in descriptions than is the case when you actually watch it happen.
Joe Lewis did a wonderful job in turning the weaknesses of his lead actors into strengths: Dall's inherent cockiness, which is often so jarringly one-note in so many other contexts, is kept in check by what had to be a decision to play off the sexual fireworks and the on-the-edge-of-over-the-top volatility that Cummins brings to her role. A more "up-to-date" version of this tale would focus on the sexual frisson, and show more of how Dall's "command" is still mostly due to an erotic bonding, something that Lewis makes sure we get (Cummins, her bathrobe, and her legs, which in later years would be more explicit and repeated several times for extra titillation) and then retools into hysteria and desperation. Cummins is mostly one-note, but this animalistic portrayal is tempered by Dall's odd passivity and comes off as more nuanced than it actually is due to the blending of their interaction.
This is what creates a sympathy in the audience for the lovers, and ties into the glamorization of criminality that seems to prop up so many of the at-or-near-the-top noirs in America. I'm not nearly so convinced now that we should exalt this particular manipulation as I was back at the time of Noir25 poll, probably because the "inner criminal" impulse that Foster Hirsch so neatly coined is, at bottom, a form of glamorizing self-destruction when there are more realistic, more complex (as opposed to complicated), more existential, more socially significant forms of human contradiction that come into play in the very best of these films, where such impulses are more surprising, more solidly built on the nuances inherent in the collision of chance and fate, and richer in their exposition of the contradictions in human nature.
In terms of ranking noirs, I think I'd try to evaluate using three basic criteria:
1) Creation of tension, conflict, suspense and its carry-through (the "thrill ride" function);
2) Demonstration of "craft" (mise-en-scene, effectiveness of actors, editing, skill in the interaction between plot and screenwriting);
3) Degree of thematic depth that is able to emerge in the film, particularly in how 1) and 2) combine.
For GUN CRAZY, I'd score these three as 9, 9.5, and 6.5, respectively. From what we generally want in a noir, it scores high (1 and 2 averaging to 9.25). From what we want in a film that goes deeper into life issues and themes to think about once the thrill ride is over, it is arguably a lesser work (2 and 3 averaging to 8). I think all five of these rankings help us go further into what makes a film tick, and having all of them on hand would guard against one "gestalt" dominating at the expense of the others.