What is a Space Opera?

I ask because I am reading Alastair Reynolds' 2000 novel Revelation Space, and that is how it is being touted.

Apparently that label was not originally a positive one. Wikipedia states that it was originally applied to "the hacky, grinding, stinking, outworn space-ship yarn, or world-saving [story]". It is more generally applied to stories where "settings, characters, battles, powers, and themes tend to be very large-scale".

OK, I assume the second is the reason it is applied to Revelation Space. It's taking me some time to read it, and it was a little slow to get into, but now that I am, I want to see what happens. I will be reviewing it at jupiterthree.com when I finish it.