I would like to echo the other comments for the need of this feature.  Having been an eXtreme Programming style agile practitioner for over a dozen years, I find the lack of a "User Story Headline" to be almost a showstopper for using your otherwise outstanding service.

All the other features that look like additional fields to user stories can be argued as more or less critical depending, on one's particular flavor of agile; however, the "headline" idea, in addition to the user story ID, is what enables most conversations around a user story to have a context.  It's the name we use to "hang the conversation" on -- I hope that makes some sense.

I love how ID's can be converted by changing the prefix for the theme.  I love how the tabbed completion mechanism streamlines the ability to write a story fast.  Basically, I love it all.  I the only hitch is the lack of headlines.

One more side comment.  With the addition of a headline field, quite a few of the other suggested features could have work arounds by simply following an convention in the headline usage.  For example, suppose one has a client that needs an "epic" identifier for some cross section of stories, a story headline might be "[EPIC: Security] New security role for team-leads" and so on.

Thanks for create a great product.