Over at Mark Rosenberg has published a tutorial on Using Outlook Without Folders, Without Filing.
This is so much un-GTD that I had shivers going down my spine when I realized what this was about. This is also likely targeted at those lucky among us that don't fear seeing an inbox full of 12,345 messages — then waiting for them to get sorted, the way author suggests.
I have known people to go by "my inbox is the place I keep all of my messages" approach — but I could never equate them with the most productive folk. True, Mark does suggest a few enhancements to how you keep all this info in one big pile: color, use different font, etc. Yet you're likely to go crazy if you do need to manage too many tasks/projects/relationships this way (say, is bold-italic-lime related to that mega-project or not?). This approach does not really help you organizing your info, it only reduces that 15-second decision you should have made when you got your message (and if you remember, David suggests that you should never file it in multiple places, you should rather go with the folder that first comes to mind, that's it).
Folder-wise filing, with a very quick heat-of-the-moment filing decisions coupled with a good search tool like (no longer available for download) LookOut — this is what does make your mind flow, not a big pile of info.