It's probably a fool's errand to review a book that's 64 years old, and has been read by everyone who has an interest in science fiction, no matter how slight. It's like me stumbling across Half-Life or Portal and going on to rave about how they're awesome, ground-breaking games: Who am I talking to who doesn't already know it? It's even a re-read for me, since this was the first series in my Unfinished Series project that felt significant enough for me to read the entire series over again, instead of just reading a summary of the volumes I'd already read.
Foundation is an excellent book, for several different reasons. First, it's an incredible idea, well realized and well implemented. Second, it's a complicated concept that doesn't require a lot of re-reading of sections in the book to understand what's going on and why. Asimov doesn't have to oversimplify anything or go into great scientific detail to get his points across. Third, it's a gripping story with a growing suspense, not just in each of the five parts of the story, but across the entire story as a whole. It's an amazing achievement that Asimov was able to put all of that together and make it work so well.
What makes this book such an icon of science fiction is how Asimov so brilliantly depicted the rise of one empire as another one fell. It wasn't one single thing that caused either effect, but a series of events that were scientific, religious, political, and economical; in short, it was a series of the same kinds of events that cause any empire to rise or fall. A lot of history went into this book, and that it's shown with a minimum of effort (the book is less than 300 pages, and covers nearly 200 years of the Foundation's history) shows just how well Asimov did it.
I've read a couple of comments here and there about this book and Asimov's style being a little dry, but I didn't see it while I was reading the book. There were moments where it felt like he was showing more than telling, but then I would find myself flying past the next section of the book, carried by the dialogue (which comprises most of the story). There were other times where the characters seemed like they weren't fleshed out very well, but then I realized that each character in the book had a feel that was distinct from the other characters in the story. No one character felt like a carbon copy of another (though Seldon and Hardin overlapped in some cases), which was quite an achievement, since each individual part of the story featured different characters.
Foundation is easily a five-star book, worthy of reading and re-reading as necessary. It moves swiftly, gets you thinking, and tells an entertaining story from beginning to end. I'm not at all surprised that this book is considered a foundation (ha!) of science fiction.