First off, if you ignore the whole of the rest of the post, please at least listen to this:
Please give us the ability to stack files when we are in playlists or playing now. It's easier to tag photos once you bunched them into a playlist sometimes, and the lack of stacking ability is very limiting.
Also, how about a "Stacking" action window with all the stack related tools in it. These could include:
1. "View stack" (shows thumbnails of each file in the selected stack)
2. "Stack" (stacks selected files. could have ability to move certain thumbnail to the top to make it the stack cover)
3. "Unstack"
4. "Expand/collapse selected stack" (the currently expanded stack would have a different color binding box to regular 'selected files' to help make it clear which files have been expanded)
5. "Expand/collapse all stacks" (the icon for this tool changes so that the user can see whether stacks are expanded or not without having to click on it.
6. "Auto stack files by name" (this action window would show a list of file types that are included in the selection (all files if none are selected) and allow you to order them in terms of priority. Eg, If there are flac, mp3, wmv, and wav, you could put flac and wav at the top so that they get made the stack top files whenever they are found)
All of these tools exist in rudmentary form. Hopefully MC13 is going to be the version to make them shine, and introduce users to their benefits.