When doing ":e /var/<tab>", Vim will pause for input after the first match, eg. "/var/X11/". Further presses of tab will scroll through the other directories under /var. In order to go through the files under /var/X11, however, one could have, at the point of ":e /var/X11/":
* Type '/' and then tab, resulting in ":e /var/X11//<first file>".
* Type <ctrl-d><tab>.
The latter results in a tidier command line because it doesn't show a double forward-slash, but a messier main window, because all /var/X11's files are listed.
I know having a double forward-slash is a minor thing, but it makes me feel that there's another keypress I don't know about, that tells Vim to start looking in this directory -- is there?