Folders
1) Allow more than 13 subfolders within a folder. 2) Add support for moving folders into subfolders. 3) Remember "Views" folder setting - show or hide. 4) Indent subfolders a couple more spaces - currently it is difficult to tell folders from subfolders because it is only indented one space.
1st
ranked

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Anonymous commented
I vote for Item 4 - Indent subfolders more (they don't look indented at all to me). Better still make a colour or font weight difference between the two so that sufolders can be quickly recognised as such.
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C.R. Tempelaar commented
You can move folders with Thunderbird. Yahoo Mail synchronises that. A method of poverty but it works.