I started reading this series after seeing all the recommendations tagged on to Wheel of Time reviews, and at first was disappointed. The prologue... been there, read that. And the book does start off slowly. The good news is that the pace picks up, towards the end of this one and in the sequels--by the time I finished A Storm of Swords (the 3rd book, and my favorite) I was in love with the series. I didn't think I would want to reread A Game of Thrones, but now I have, and found it much better the second time around, in the context of the series as a whole. The thing is, this book is mostly setup; well-written and engaging setup, as you can tell by the multiplicity of 5-star reviews, but still setup.
As far as what sets Martin apart from other fantasy authors... well, there's the dialogue. He knows how to write it. And it's often funny. Believe me, you start to need the comic relief. And there's the way he subverts expectations... several of the characters I expected to be major villains in this story either died unexpectedly early or have morphed into heroes. Then there's the way he assumes an intelligent readership. The backstory, instead of being handed to you, is something you have to piece together, and the answers to big mysteries are not obvious. And there's the political intrigue, which deepens as the series goes along. Martin knows how to write this stuff. You won't see any Robert Jordan-style conferences with women sitting around smoothing their skirts and glaring at one another... in Martin's world, politics means raising armies, arranging marriages, ordering assassinations, etc. He's well-grounded in the historical period as well: wounds fester, they leave scars, and so forth. No magical shortcuts here. Magic in fact is quite understated here, although there is more in future books.
There are a couple misconceptions I would like to counter, though. One: don't be scared by what people say about the death rate! Yes, there is a lot of death in these books, as there was in real medieval wars. However, very few "viewpoint" characters (the ones with chapters named after them) have died so far, at a rate of less than one per book. That's not bad when we have 14 of them by book 4. Occasionally a character you care about will die, and brutally, but Martin's been quoted saying there are several he cares about too much to kill. So don't worry, some will survive! Reviews on this site actually led me to expect far more death than there was, and to assume characters dead multiple times, only to see them return. If most of your reading (like mine) consists of standalone, non-fantasy fiction, you probably won't be too shocked. Just don't expect every important character to survive everything, as they do in most epic fantasy.
Two: the "gray"ness: there are people of real integrity in these books, as there are brutes who revel in cruelty. As well as a lot in between. The thing is, Martin's good guys have to make a lot of tough decisions, especially those in positions of power; this isn't a "just be virtuous and everything will fall into place" sort of world. There are still characters who try their best, though, which to me is far more admirable when it's not easy.
And a note for the historical fiction fans: yes, there are two families here named Stark and Lannister (kinda sounds like York and Lancaster) and a deadly feud between them. And here end the similarities to the War of the Roses, as far as I know. Expect a well-researched, realistic medieval setting, but don't expect a retelling of British history with magic thrown in.
As other readers have pointed out, these books aren't for everyone: the atrocity count is high, the scope and cast are huge, and Martin hasn't even finished yet. My recommendation, though: read it, and if you're discouraged at first, keep reading! It only gets better from here.