Along the lines of my proposed "merge tags" feature, creating stacks using tags would be extremely helpful for quickly forming a stack to begin organizing thoughts.
For example, I might want to begin organizing my ideas about Napkin's beta into a stack. I would want to create a stack containing all my notes tagged "napkin" and begin sorting them into sections. Or, I could add multiple tags (such as "napkin tagging," "napkin searching," and "napkin stacking") and create a stack already organized into sections of these tag names.
This could be done simply in the stack overlay, where user can add groups of notes by tag into stacks. Or, it would be cool to utilize a multi-tag filter that you are working on to create a stack from the search overlay.