Trello is the always going to be a horizontal app. And sometimes people will need to move to something else to solve their problems. If you are using Trello as an ATS and you start getting 50 applications a day, you are going to go nuts. You need to switch to Jobvite or Greenhopper. If you have a team of 100 devs trying to track your bugs, you want to use FogBugz or Jira or Pivotal.
Joel used a metaphor that I like: The verticals are like stones on a beach and Trello is the sand that fills in all the space between.
In the future I think you'll see the Trello product evolving to support those other verticals and adding value (i.e. quick overview) in the way Trello does best.