I use a couple of GTD tags like "waiting for" and "someday", then use Gmail's Multiple Inbox to keep them in peripheral view so I remember review them periodically. Otherwise search is the way to go.

Gmail shortcut tip: when looking at a message, type "l" (lowercase "L"), which will popup the labels picker, then type the first letter of the tag you want to apply, then enter to apply it an "y" to archive.

Yeah, a lot of people don't tag things. Search is really paramount for any application that's geared toward productivity or communication.