In OWA and Outlook, we're always interested in how people manage their inboxes - how do users handle email overload? A few main strategies came out of our usability studies - that users tended to be "Pilers or Filers". Pilers tend to let all their mail sit in their inbox, and they operate on it there. They like having it in one place for quick and easy sorting (and type-down search). Filers tend to file mail into separate folders - perhaps based on who it's from, or what action they need to take next, etc.
When OWA comes into the picture, it adds another twist - how a user manages mail in Outlook might not apply to OWA, since they might tend to use OWA for a quick scan of the inbox from home before going to work in the morning, or reading email in the evening, or a few minutes of email at a public kiosk while waiting for a meeting to start. One thing that we came across is that a lot of people wanted the ability to mark items as unread in OWA, because they used the 'unread' state as a way of tracking "the things I need to deal with once I get back to my office."
So in OWA 2003, we enabled mark as unread as well as other common tasks through keyboard shortcuts. This is one of my favorite features in OWA 2003, being the keyboard junkie that I am!