Confusingly, gcc accepts -Wunreachable-code as a valid option, but then proceeds not to warn anything. (edit): Which is a known bug apparently,
I've been wondering how hard it would be to get to the point where the defaults are rigorous and developers have to opt out with specific -Wno-… options.
We have it turned on, and about 10 warnings specifically disabled because they were too noisy or not useful for us. It's always interesting upgrading Xcode and seeing what new warnings we get to fix.
My other beef is that compilers always add new warnings as options or behind new "catch all" flags like -Weverything that no one knows about. As long as each new warning can be individually disabled, there isn't a huge cost to pay by making much more of them enabled by default. Upgrading to a new compiler version usually requires a tiny bit of work, so adding a few -Wno-* rules for new things you want to disable until the code is clean (or forever) is a small price to pay for all new code getting the checks by default.