We've been fans of Gmail's Priority Inbox feature ever since it launched, but it was sometimes hard to tell why Gmail marked a message as important. It turns out that Priority Inbox tells you — you just need to show the message's details.
If you open up a messaged marked Important and hit "show details", you can actually see a line of text explaining why that message was marked as important. Usually it's something like "the people in the conversation", "because the message was sent directly to you", or "because you marked it as important", but it's still pretty nice to know — especially when something shows up in your Priority Inbox that shouldn't necessarily be there. You can also get this information by hovering over the "important" icon, both in the message header and in the inbox view (as shown in the image to the left).