I read it for the first time in college, which would be almost ten years ago, and I really liked it. I became a big fan, hung out on the message board, chose a picture of Robert Baratheon killing Rhaeger Targaryen for my desktop, and so forth. A few years ago I went back to read it again and found myself enjoying it a lot less. It may simply be the case that at the first reading I'd barely read any other fantasy except the Harry Potter books, while by the second reading I was much more aware of what was out there. Some of the things I noticed were:
The series is jam-packed with gruesome violence and torture, especially in the most recent volumes. There's something distrubing and disgusting going on in nearly every chapter. Maybe the author thinks that this stuff is supposed to be shocking, but to me it fails to even reach that level because it becomes so repetitive. I find I can only read about a man being forced to eat his own severed body parts so many times before it just becomes irritating.
The series is, for the most part, usually predictable. Some people say that Martin is very free with killing off characters, but actually the major characters such as Tyrion, Arya, Jon, Sam, Sansa, Daenerys, and Bran keep escaping from death by far-fetched coincidences and deus ex machinas again and again.
The foreshadowing and symbolism are extremely heavy-handed and virtually impossible to miss.
The number of characters and vying forces is far too large to keep track of. I feel like the amount of effort I'd need to remember who's who would be equivalent to what it would take to learn quantum physics. Some people think it's a good thing that the series is so huge. If you like it that way, help yourself. I personally prefer shorter books where the author achieves an elegant effect with a small cast of characters.
And lastly, the complete lack of any moral vision. The series if full of groups that are utterly barbaric and depraved, such as the Iron Men, the Dothraki, and so forth. Yet Martin's attitude towards those groups leans towards a politically correct, "they're not bad only different" attitude. Nor does he offer a good portrayal of those characters who are fighting to defend civilization; indeed, much of the series suggests the belief that civilization isn't really worth defending.
Just my two cents, of course. And I'm not watching the series.