Might also want to note :cq[uit] to quit with an error code. This can often be useful when you're using vim as your $EDITOR and wish to tell the calling process that you want to abort.
Oops, I probably could've offered examples. So, when writing a mail I can choose to simply give up instead of dropping me in to a dialog for "save draft/quit?", or to drop out of an edit session in `git add --interactive`, etc. The reason it can be useful in these specific instances is that you don't need to unwind your changes if you've run filters to modify the text for example.
