Some people organize their tasks with a small set of tags to indicate state of a task or project. For example #next, #today, #soon, #later
Feature suggestion: Add a (presumably advanced / paid) feature that allows a user to assign shortcut keys to tags. If the focus is on an item or items and the shortcut key is pressed, it adds or removes the given tag on the item(s) selected or with focus.
A related feature would be to do this assignment / toggling via the command palette. The command could be "Toggle tag", and would after being invoked prompt the user for a tag to use.
Note that the two features are complementary but are not the same. In the case of direct shortcut assignments to tags, a user can extremely quickly and with a single keystroke toggle a single tag. The second feature really is more of a bulk update that would apply any given tag or tags to a set of items.
Feature 1 use case: While composing or modifying a set of items, a user can treat tags as another attribute of an item just like priority or star. He/she can quickly tag items without needing to type the tag perhaps multiple times.
Feature 2 use case: Bulk updates.