I would love to be able to select an MXD in ArcCatalog and instantly see the layers that are used in the map. It could function the same way as a folder. I often want to make sure I use duplicate symbology and layer properties in two different MXDs but this means I have to open the source MXD, copy the layer, and paste it in my destination MXD. Why can't I just drag the layer out of the MXD? That would be cool.
This idea has been merged with Display and manage Map Document contents in ArcCatalog.