I think this film is really amazing for two reasons. The first is fairly direct and formal, having to do with how Resnais takes what is essentially a piece of literature from Duras and makes it cinematic. Through the use of thoughtful cuts back and forth across time, with the sound from one time overlaid onto the images associated with another to blur and merge the different temporal points, Resnais gives life to what could have been a talky, pretentious disaster.
I think it is particularly difficult to make long monologues/dialogues cinematic, yet Resnais has done just this. His juxtaposition of horrifying images with sensual ones, transforms the film into something that draws you, inexorably.

The second reason is hard to describe. Hiroshima Mon Amour can still amaze, provoke and evoke because of the ambiguous connections Resnais/Duras make between remembering, forgetting, and forgiveness. The first two concepts/ideas are pretty clear and many others have written about them in connection with the film. Yet I feel forgiveness must also be there, floating in-between remembering and forgetting.

There seems to be a paradox in human existence for Resnais. How do we remember and forgive? How do we forget and go on without the risk of being caught in creating precisely the same horror we have resigned ourselves to forgive ?

This film cuts right to it because the most tender, caring emotions always seem to require an unforgivable act in order to come forth. Almost nihilist.

Duras has written, "Why deny the necessity of remembering?"
Yet this remembering obstructs forgiving, forgetting, living.

A bit of dialogue from the Hiroshima Mon Amour (Duras' script, translated by R. Seaver) however, puts a lot more into perspective.