I'll do a little comparing of Martin to Tolkien. I can't believe my arrogance in attempting this... well, actually I can believe it. I'm Boaz, I'll jump at any chance to come off as foolish. Of Tolkien's works, I am very familiar with The Silmarillion, The Lord of the Rings, and The Hobbit. Yet, I've only read four of Martin's books... AGOT, ACOK, ASOS, and AFFC.
This is not meant to be an exhaustive or complete comparison. I'll try and discuss just a few areas. But, let me add this caveat before I begin... I devoured Tolkien at age 14, but did not pick up Martin until I was 35. So I am judging these authors by my initial reactions at very diverse ages and judging them based upon rereadings... Tolkien over twenty-five years and Martin over five.
One reason, and it's a big reason, I love Tolkien is that he put so much effort into his work before the public ever glimpsed it. True, he always tinkered with the stories that eventually became The Silmarillion and so they never became completely cohesive, but the scope is truly tremendous. Tolkien's son and friends read his works before he submitted them to his publisher.
Tolkien's effort included not one, but at least six languages... none of them are complete (all are fragmentary), but the feel of emotions and culture resound within those words. By the way, none of the languages use the letter X... in my opinion authors use X to describe alien or exotic places and names when they don't have a clue about how the alien and exotic should really sound.
Martin has not attempted to create languages for ASOIAF, except (iirc) for the few words of Valyrian. Valar morghulis, valar dohaeris, dracarys, and valonqar are the few words Martin gives to us. Valyrian compares to Tolkien's Black Speech, languages of the Woses or the Rohirrim in depth. But I do not chasten Martin for this... he is not working as a linguist. But I've thoroughly enjoyed Martin's slight changing of english spelling to create very real sounding names. Eddard, Sandor, Rickon, Loras, Robar, Sansa, Sandor, Amory, and Barristan all seem English with some French or German influences. Also, Martin only has two characters with X's in their names that I can recall... Jalabar Xho and Xaro Xhoan Daxos (sp?).
In my estimation, both of these authors both knew where they wanted their stories to go from the very beginning. They knew the events. They knew the dilemmas their characters would face. They knew the important decisions that would be made. They knew the themes they wanted to delve into. In contrast, Edgar Rice Burroughs' work is pure pulp. He seemed to have kept cranking out stories as long as they were selling. (By the way, I love John Carter of Mars.)
Tolkien's world was intended as a sort of precourser to our own history. He was writing myth to fall in line with our world. But he created his own special world with special creatures. Martin's world, on the other hand, is not a precourser to our time nor is it an alternate world. But England, during the War of the Roses, is obviously implied.
Both have taken our histories and myths for their inspirations. Tolkien's creation story echoes the Genesis chapter one account. Numenor is, or at the least strongly implies, Atlantis. Boromir's fight to hold the bridge at Osgiliath is straight from the Roman legend of Horatio. Martin uses the War of the Roses... York becomes Stark, Lancaster becomes Lannister, Tudor may be Targaryen, and Neville may be Tyrell. The Ironborn, the Dothraki, the Wildlings, and The Wall all come from templates of our world.
I think the main differences between Tolkien and Martin lie in style of story telling and in the message.
In The Hobbit, Tolkien keeps a very light tone. The target audience is adolescents and adolescents at heart. In ASOIAF, Martin does not target adolescents, except mayhaps when he just tries to shock his readers.
In The Silmarillion, Tolkien attempts to present his stories as being related by a master bard. The characters are from ancient times... and they stay that way because all of the tales are told in the past tense. The only past tense that Martin uses, iirc, deal with Ned's dreams.
Perhaps the biggest difference in style is the use of hobbits as the only storytellers of LOTR versus Martin's use of different character's POVs. By my count, twenty-five different characters are used by Martin in the first four books to present his epic. By using all these characters, Martin is able to bring in various and differing emotional, theological, and psychological reactions to events... sometimes to the same events. From this we get the ethical values of the characters. In contrast, any action not witnessed directly by hobbits, LOTR, is presented to them in a story. The one exception is the chase of the three hunters from Amon Hen to the the Hornburg. All information is presented from the ethical values of hobbits... we have to read into the text to find the real Elven, Gondorian, Entish, etc. judgements on the events in the story.
Martin is an intricate story teller. He weaves many, many characters and plots to create a multi-level tale that engrosses his readers. The political intrigue, the constant bickering, the diplomatic deceptions, and the omni-present threat of death help me to suspend my disbelief at the right times and to follow the characters' motives. But with all this going on, I feel that Martin is not concerned about an overall message as much as the overall feel. He likes to shock his readers. Sometimes I get the feeling the shock is only for shock's sake.
Tolkien, imo, is more concered about conveying a message than presenting theatre. I know Tolkien eschews allegory, but the obvious espousal of Christian theology through certain archetypes is hard for me to not descry from my vantage point. Anyway, politcal intrigue is not a plot device that Tolkien uses much.
But for me the biggest overall difference between these authors is the presentation of their theological and moral beliefs on leadership, community and God. And these are probably best left for another post... this one is long enough.
] yet the stories have this unique quality to them that when read do not come across as silly or stupid which is the way I felt about Tolkien when I first read his work. 
