Allow the creation of Categories/Groups for Lists
Right now all of the lists are grouped together alphabetically,regardless of their association with each other. I think it would be an improvement to allow the user to create categories/groups that the lists could belong to and be grouped by. It could be done either with a tagging system for adding members to a particular category or explicitly allow the creation of categories that lists could be dragged into, or otherwise be made a member of.
This could be toggled on or off by user preference.
As for the reason for this feature, consider when you have multiple lists that are for related projects, how useful it would be to see at a glance all of the lists belonging to that project type. It helps avoid confusion, and also helps avoid having unneccessarily long lists that are related by type, but not by purpose.
To give an example, imagine you're a writer who has a three-step process with corresponding lists for Research, Rough Draft and Final Draft. You don't neccessarily want just one long list of tasks, and you also don't want to see your Grocery List and Reminders List crammed between these on account of alphabetical ordering. This would help with that workflow.
DESIGN As far as design, I was imagining that you could change the current "New List" button just to "New" and under that have options for "New Group (or Category)" and "New List".