It was Ray Bradbury, along with maybe a couple of others like Kurt Vonnegut and Theodore Sturgeon, who were the first to approach contemporary science fiction with the same literary standards that applied to any other sort of fiction. The effect of this approach on a generation of writers cannot be overstated.
I never had the pleasure of knowing Bradbury, but he was an amazing writer. We all know that when we write fantastical fiction, we are, in fact, writing about the here and now; but it always seemed to me that Bradbury was more keenly aware of that than most. He wrote stories about Mars that were about the way human beings interacted; he wrote about living tattoos that were stories about kindness and cruelty.
Yes, he wrote Fahrenheit 451 against censorship. But the point is that he wrote it in 1953—at the height of McCarthy’s blacklists and red-baiting. At a time when writers and artists were falling all over each other to establish their anti-communist credentials, Bradbury was one of the few to take a principled stand.
He was also famous for being helpful and supportive to new writers.
I think there are others better able to discuss his effects on the field; but it’s safe to say that literature in general and science fiction in particular would be weaker without his work.