This classic is filled with a lot of memorable images - from the opening scenes on the seashore to the effective concluding shots, creative thoughts are combined with some fine camera work. There are several significant or interesting questions raised by the characters - from the imagery of the "Seventh Seal" in Revelation, to their simple but important concerns about eternity - but it is the way that the visuals play off of the ideas that make the movie so worthwhile.
The recreation of the medieval world is convincing and effective, with a lot of detail to set off a varied assortment of characters with different personalities and perspectives. The characters are not necessarily very deep, but most are interesting, and are worth caring about. The ways that they deal with their discouraging situation make you wonder what it would have been like to live in their world. It's also a movie that in some respects is even better to watch over again, after you already know what has happened and can then pick up even more of the detail and imagery.
No doubt the somber tone and slow pace will always keep it from being widely popular, and it's not perfect, but it's satisfying in a different way, and deserves its reputation as a classic.