I found this bit from Knapp in an old Classical Weekly. Though I disagree with him somewhat, he makes a fair point:
"We are not satisfied with Book IV, as a whole, simply because we do not believe in the gods. When we are not listening to Dido, we are thinking of Aeneas: at Jupiter, Juno, Venus, and Mercury we glance dourly over our shoulder when they speak, and forget them utterly when their words are ended. But they rule the action. Could we realize their existence and power as vividly as Dido's love and despair, our verdict on the poem would be altered completely. And here lies Vergil's vast failure―his one vast failure in this Book; he has not succeeded in making us believe as we read that Juno and the rest are even more real than Dido―and no less than that (one writes it with all respect) it was his plain business to do. We do not believe in Zeus and the inspiration of the Delphic oracle, but while reading the Choephoroe we experience all the emotions which Aeschylus intended to arouse, not simply a horror of matricide. The weakness, then, of this Fourth Book is certainly not that Aeneas acts shamefully, but that Vergil, having pinned his every chance of success to our belief in the gods, has failed to produce that belief in us effectively."
"We are not satisfied with Book IV, as a whole, simply because we do not believe in the gods. When we are not listening to Dido, we are thinking of Aeneas: at Jupiter, Juno, Venus, and Mercury we glance dourly over our shoulder when they speak, and forget them utterly when their words are ended. But they rule the action. Could we realize their existence and power as vividly as Dido's love and despair, our verdict on the poem would be altered completely. And here lies Vergil's vast failure―his one vast failure in this Book; he has not succeeded in making us believe as we read that Juno and the rest are even more real than Dido―and no less than that (one writes it with all respect) it was his plain business to do. We do not believe in Zeus and the inspiration of the Delphic oracle, but while reading the Choephoroe we experience all the emotions which Aeschylus intended to arouse, not simply a horror of matricide. The weakness, then, of this Fourth Book is certainly not that Aeneas acts shamefully, but that Vergil, having pinned his every chance of success to our belief in the gods, has failed to produce that belief in us effectively."