It's just a genuinely good story. The books are just enjoyable. They're long winded, and sometimes go on and on forever.
But that's because he's building a whole, detailed world.. There's a lot of material there, just like LotR. The good thing is, they have plenty of action and give you a small basis to start with, and expand the world as needed. You can salivate over luxurious meals they're eating, or frown at how disgusting and real the medieval poor sections of a district are. You fear for the lives of characters, and it really is a strong emotional journey. There's a character in that book for everyone--and many of the principles, and morals apply to modern day life. The back stabbing, the fact that some people are just bad people and keep getting rewarded for it.. the fact that justice shines through sometimes.. that karma may exist--or not.. that bad things happen to good people. The mysteries and puzzles to solve, the theories you conjure trying to make sense of events--and by the time you get an answer, it feels like it's far in the past, and there's more mysteries to unravel. It's a strong story.
And the show does not do such a bad job of summarizing thousands of pages and man hours into bigger-than-life feeling, believable scenes and settings. Seeing characters come to life, and watching them walk and talk.. The series is just far more enjoyable to watch after the books have been read. But it isn't at all unpleasant for those unwilling to read. You get the jist of things, and you can have nerdy book-loving friends fill in those gaps for you otherwise.
There's so many nerdy things you can indulge in. You can nerd out to the cookbooks and medieval-centered dining. The costumes. The principles and morals of different houses and characters. You can gossip about what you think will happen to x or y character or house. Who will be in the throne next. It's easy conversation starters for socializing, and can become indulging-ly deep without fearing hurting people's personal morals or opinions because the morals are that of another world's entirely. There's magic in there for magic lovers, and theres hard core realism for those that like no safety nets.
I don't think the actors or the producers thought it was going to get so big. I watched an interview where they made a mention "Look, this isn't really something that gets you emmys or anything.." but it exploded. The advertisements are really awesome--and the attention to detail they put into things is really important. There are no lame "printed to look like x" costumes. There's a lot of effort there--and it makes a quality product. The actors are good at what they do.
There are some downsides--it's involved.. and clearly so. So if you aren't a huge fan, yeah, it'll seem a little slow paced and boring to you. It's a little difficult to get the characters straight because, let's face it, they have to cram thousands of pages into hour-long episodes, so if you're a casual watcher and not really paying attention you can get lost in the plots and who is who character wise. It is gore-y and dramatic.. no one, not one character, is safe from death. That's a well known fact even if you never hear a single spoiler ever.
And, honestly, the series is only like.. 3 seasons in, with the 4th just premiering yesterday. So, there isn't a ton of information to catch up on. It's comparable to other series lengths that don't have all of the hype this show does.