What would I like to customize?

-text in sizes and fonts that I find readable. Two choices--serif & sans serif--is not a choice.
-readable contrast between text, background and Yahoo's dreadful, irritatingly loud color and pathetically minute color/theme selection
-a traditional nested organization for my folders
-let me turn off/delete Yahoo's silly 'views' & please stop subjecting me to stupid, prejudicial and worthless algorithms designed by teenaged programmers
-ditto the 'personalized' ads. Ads based on my interests? Okay. Ads based on mindless prejudiced algorithms done by insular conformist programmers. Wrong!
-Adding flexibility, stop taking it away Bring back context menus. Cluster the inline icons for move, delete, trash -- this grope all over the line for previous functionality is a pain.
-make the sidebar adjustable, and return the nested/indented directory structure.

e_mail is about usability. Yahoo's missed the boat on this iteration.