I was browsing the stacks at my local library and was glad to find on the shelf The Space Opera Renaissance. It’s a big, fat, honkin’ book of stories, starting way back in the early twentieth century. The editors supply a lot of historical context, which is fascinating. They make much of the fact that space opera was originally purely perjorative; they seem to want to bring back the term "space fantasy" for the good, non-hard SF stuff that emphasises the heroic over the scientific. They explain how some writers (like E.E. "Doc" Smith) were originally viewed as hard SF writers, and only later became seen as belonging in the space opera category (this happening around the time the term space opera was rehabilitated, which was the work of Lester Del Rey and his wife–what’s her name).
The early stuff, even at its best, is a mixed bag. Consider The Prince of Space by Jack Williamson. He wants a space ship to have a force field, so he throws around the word "vibratory" to make it seem hi-tech, but it’s obvious he’s pulling the idea out of one of his orifices. Later, he describes a huge cylindrical space station that rotates to create artificial gravity for the city built on the inside. This predates Clark’s Rama by quite a bit, and is believed to be the first time anyone described such a thing — very impressive. (I’m always a sucker for large-scale hardware anyway, like Rama, Greg Bear’s Ion, the space elevator in Red Mars, and the granddaddy of ‘em all: Ringworld. I’d love to visit any of those.)