I have a lot of feelings about this, and I'm not even sure how to articulate all of them. I haven't read everything he's ever written; we had Fahrenheit 451 in high school, of course, but I liked Something Wicked This Way Comes a good bit more. I actually just reread The Martian Chronicles and Bradbury Stories last month (the latter isn't the Compleat Works of Ray Bradbury, but it's the biggest collection I've found). But when I was in high school, the school paid for a few interested students in my writing class to go to the Writing Today conference to see him speak. He did what I think was supposed to be a Q&A, but was mostly just him rambling on about anything he wanted for more than an hour, because that is a precious gift when you're Ray Bradbury; he was also the guest of honor at the luncheon, but I remember the morning session more clearly. Actually, since this was more than fifteen years ago, I only remember two specific things he said:
1) Never throw away the things you love, and never let anyone else throw them away for you. He still missed his childhood Buck Rogers comics. Since I had just thrown away a pile of clippings I had cherished for several years, this was resonant, validating, and bittersweet to hear, all at the same time.
2) Never talk about what you're writing; you won't feel the need to write it anymore. Even having heard him say that, sometimes I can't help myself. But he's always been right.
As for the books themselves, his real thematic passions as a writer seem to have been 1) the idylls of small-town childhood; 2) the joys of books (obviously), both as the building blocks of the soul and as something to be defended at all costs, and with outright rebellion if necessary; 3) and the magic of the rocket age. (There are others, but those are the ones that strike me the most strongly off the top of my head. I might throw in "the need and search for love," as well.) Particularly that last theme--so many of his short stories are about space travel, the idealized glamour and possibility of rockets, a longing for the new-world opportunities they offer, and sometimes even an anxiety about what they might herald. Bradbury's work embodies an era of sci-fi that's... really kind of past-tense now; I think it was superceded by the "space colonization" and "omg aliens will eat our faces and destroy the earth" genres, because we moved past the hope that we would someday land on the moon to the dread of, "Oh, shit... what next?" The modern age--well, let me back up. Every age is the modern age to the people living it. The hopeful, post-WWII Rocket Age was the modern age then. The next modern ages were colored more by disillusionment and anxiety, as far as I can tell, with upswings now and then. But the horror stories people have been telling the last few decades usually aren't about Unstoppable Blobs and Brains That Would Not Die and Creatures What Came from the Deep that cheerfully get vanquished by small-town heroes in the final reel. Even in, say, Independence Day, half the world is lovingly incinerated before the various damaged characters come together to redeem themselves and save the Earth, etc., etc., but you know, with those final shots of smoking crash sites, that civilization isn't quite going to be the same afterwards. It's going to have to cope with the knowledge of what happened, what turned out to be possible, long after they've rebuilt New York. (Honestly, Spielberg's War of the Worlds is one of the most traumatic movies, for me, at least, that I have ever seen, because it so viscerally tells you exactly how screwed we all would be.) People have to come to grips with the horrors of time and space in post-Rocket Age sci-fi; in a timely enough fashion, this is what the Alien movies, and I'm guessing Prometheus this weekend, are about.
But in Bradbury's cosmos, the gleaming rocket is a symbol of hope and opportunity--opportunity that humankind often wastes or destroys, humanity being what it is, with only the Jeff Spenders of the world to rail helplessly against the inevitable. On the other hand, you have the very pointed "Way in the Middle of the Air" (1950), for example, a story about African-Americans taking rockets to escape... well, the America of 1950, and start new lives on Mars, on their own terms. (In fact, I would say "people seeking to live on their own terms" is another big theme, and one often interwoven with the others.) As cynical as many of Bradbury's stories are regarding what a mess humans will make when they get to Mars (and of Earth itself back home), those stories are about hope wasted, which presupposes that there was hope and joy and opportunity in the first place. I feel like his writing wouldn't be so vivid and lyrical if there were not such a wellspring of old-fashioned hope at the center, so much he found worthy of protection. True-hearted rebellion that's more than just anarchy, after all, requires a world worth saving; a story like "The Toynbee Convector" posits not just that one man could change the entire course of civilization with nothing but a message of hope, but that humanity itself is capable of acting on it. (Which is, at a moment when the YA dystopia genre is huge and yet almost eclipsed by what's actually going on in America right now, a precious kind of magic.) And I think that's what his genre legacy is, a golden slice of crystallized possibility--he claimed most of his work was fantasy, not sci-fi. I would argue that they're just different ways of dreaming, and dreaming is the only way we move forward.