I find it difficult to argue with The Everett's definition of Space Opera except that it appears to be a bit more restrictive than I'd define it.
To me a Space Opera is any story containing the following elements:
These elements must of necessity
ignore the
impossibility of
faster-than-light travel, or invent some mechanism for it. Because of this, these stories de-emphasize hard scientific fact at their cores.
This isn't always a bad thing.
A large percentage of Science Fiction stories qualify as space opera, according to this criterion. Many of these are of high quality, but an author must be careful to keep from slipping into writing according to a formula.
Some high quality space operas include: