I would like the option of viewing the extended tags dialog in the tag panel instead of in a window. If this option was enabled, the tag panel could have tabs to switch between the regular and extended view (like the Opera sidebar).
You could stretch the panel out if you needed to see more of the tag contents.
This seems more intuitive to me and would be extremely useful as I use the extended tags dialog extensively.
Edit: Alternative thought - have a tag panel option to "Show All Available Fields of Selected Files" instead of choosing a fixed set of tag fields. That way the drop down list showing field values would be available for each field present in the tag if multiple tracks are selected.