I absolutely do consider filing detailed bug reports to be contributing. So much so that I have
it on my resume, under the section labeled "open source involvement". Nobody has ever
commented on it (and yes, I have gotten job offers, so this isn't a completely irrelevant
anecdote. Just mostly.)
But if that's not contributing, what is? It's often more useful than a patch, because most
patches need to be first interpreted and reverse-engineered to figure out what problem they
are trying to solve, and then usually rewritten from scratch to satisfy concerns that the
submitter (justifiably) doesn't care about.