The Tolkien comparison seems apt because God knows he was far from a perfect writer and as for unrealistic dialogue... Shakespeare could make long, poetic lines work because of his impeccable sense of timing and cadence. Tolkien was no Shakespeare.

What still makes LotR lasting and influential are the solid world-building, the truths of life in its fiction, and the enduring moral soundness of the story. I've found a work can overcome a great many flaws to be a classic (Les Misérables, anyone?) as long as it rings true and firm, while slicker, better-written works of less substance are forgotten.