I'm at page 637 now, and I thought I'd post a few random thoughts. I wanted to do that before a few times but didn't find time. If any other newbie happens to come here, spoilers ahead.
First of all, I'm really loving this book. I used to steer clear of medieval fantasy because I'm always afraid it would be all "heroic knights fighting evil overlords and rescuing damsels, and weren't olden times so romantic and wonderful" and that kind of rubbish, but I knew that Game of Thrones/Song of Ice and Fire is something completely different - brutally realistic, layered, a Medieval-type world seen through the eyes of a modern sensibility, and I won't bore you, I'm sure you've heard it all before.
I've heard about he complexity and moral ambiguity in GoT, but really, how many books can shock you with a brutal murder of a child, show the murderer laugh about it, and then make you feel some sympathy for the said murderer a few chapters later, and even kind of root for him for a moment (even if it's just against his brother who's even worse and so far portrayed as a Complete Monster)? Even though he's still an awful person, but at least he has no illusions about it.
Sansa feeling some pity for the Hound was the first interesting thing about her character. Though I'm sure things will get more interesting as she's forced to grow up and understand a few things about the world.
However, I must say I couldn't find any sympathy for Viserys. I'm sure his life wasn't easy, but he was such an annoying shithead with no appealing qualities at all. Some characters are awful people but they at least have intelligence, wit, or courage, or a sense of humor, or a certain self-awareness, and he had none. Too bad I got spoiled about his death (although not the manner of it), because it would have been even more satisfying otherwise... although I'd probably see it coming anyway, he dug his own grace.
Regarding the discussion about Tyrion, I still don't see any grounds for the claim that Tyrion is not charming in the books. Jon would disagree, for one thing. Further, Tyrion has a way with words and can make just the right promises to get the most unlikely people to help him (even if it's for something they want rather than for any special love they feel for him). And even though Bronn fought for him out of opportunism rather than loyalty, he also enjoyed Tyrion's jokes and got very pally with him even when Tyrion was still a prisoner. Yes, these people were hardly polite society

but surely it takes more smarts and skill to say the right words to the people who aren't from your social class. (It seems that Tyrion may actually take after his dad in that respect, judging by Tywin's interaction with the clansmen.)
I was glad that the paternity of Cersei's kids was revealed halfway through the book rather than being dragged out for much longer, since it wasn't so difficult to figure out. Although I was less happy that Ned figured it out through some very dubious genetics. The whole thing with Baratheons dark hair overcoming Lannisters' blonde hair every time is rubbish in terms of real world genetics. But I guess you have to make up some crap when you're writing about a world with no DNA testing.

Speaking of dubious genetics, how about the description of Cersei and Jaime as mirror-similar to each other, even though non-identical twins look no more similar than any other pair of siblings. But that's one of the few issues I've had with the book, which says a lot. And hey, as long as people are not devolving into lizards, it's OK.
I found it a bit frustrating that Catelyn (and Ned, begrudgingly) trusted Littlefinger so much. Yes, he was like a brother to her, but is it really wise to trust the good intentions of a guy you rejected, especially if he was obsessed with you? Not to mention that everything about Littlefinger screams "tricky".
The identity of Jon's mother is a mystery that I've been eager to learn from the start, but I didn't have any ideas before the last chapter I've read, Ned in the cell, which seems to be dropping some heavy hints. So, I figured early on that Joffrey's father was not his father and his uncle was also his father, but could it be that Jon's father is not really his father but just his uncle?

It would explain a lot. Familial relations in this book can be pretty complicated, heh.