Thanks to MichaelRaskin1 in #vim on Freenode I can now remove lines that don't match what I'm looking for in a file.
To try it out, let's have a testcase. You have some truss or strace output you're looking through and want to see all of the open() calls (without dropping to a shell and running grep); try this:
This is pretty straightforward: delete (del) all lines (%g) that do not (!) start with (^) open.
Boosh. Thanks Michael.