In many regards, Grossman is right. The moral and political complexities of Martin's A Song of Ice and Fire series set it apart from the much simpler world of Middle-earth where Elves are good, orcs are evil, and you know that you're rooting for the hobbits to make it to Mount Doom. And if Grossman's claim had been that Martin is a Tolkien for our age--an age of constantly shifting alliances, moral questions where absolute right and wrong answers are elusive, and a sense that you never know where you belong--well, then I would have agreed with him.
But instead, Grossman claimed that Martin is "the American Tolkien." And with that, I disagree.
It is possible that Grossman was simply referring to Martin's nationality as compared to Tolkien's. But given the point that Grossman was making, calling Martin "the postmodern Tolkien" or "the 21st century Tolkien" would have been more appropriate. Calling Martin "the American Tolkien" suggests that there is something inherent in Martin's project that does for the United States what Tolkien hoped to do for England with The Lord of the Rings--that is, create a distinctly English mythology.
I had a mind to make a body of more or less connected legend, ranging from the large and cosmogonic, to the level of romantic fairy-story [. . .] which I could dedicate simply to: to England; to my country. It should possess the tone and quality that I desired, somewhat cool and clear, be redolent of our 'air'... (Fimi 50).
In other words, Tolkien didn't just want to write an epic fantasy--he wanted to write an epic fantasy that somehow embodied his own country. And for those familiar with Tolkien's work, the parallels between the setting of Middle-earth (particularly places like the Shire) and England, as well as the similarities between the wars against Sauron and Hitler, suggest that Tolkien was successful in this goal.
I would argue then, that for an author to be "the American Tolkien," he or she would have to not only write an epic fantasy that speaks to our time, but also one that is distinctly American in some way. And because of that, Martin does not qualify, no matter how exciting his prose or how well his plot lines are starting to intertwine. A Song of Ice and Fire is set in a time and place that does not resemble the United States; rather, the multiple warring kingdoms, styles of dress, modes of transportation, and means of fighting are much closer to medieval Europe. Martin's epic does not bring a distinctly American mythology to life, and as a result, we must look further to find "the American Tolkien."
While some might argue that Stephen King is known primarily for his works of horror, I would suggest that King's ability to work in multiple genres makes him an even more viable candidate for Tolkien's crown. As is evidenced by the ever-increasing categories of fantasy literature--steampunk, urban fantasy, cyberpunk, slipstream, The New Weird, etc., etc.--fantasy is moving in multiple directions all at once. The author who is able to blend these different elements in his fiction is the one who has the greatest potential to adapt to these changes, and as a result, have the greatest influence on what fantasy literature is.
Stephen King's The Dark Tower series not only creates an American mythology, but it also has the potential to redefine what fantasy literature is. And that is what makes him the king.