A good conference should get you out of your day-to-day tactical problems, and get you focused on the big picture. It should put you cheek-to-cheek with people who are excited about being in the same space, and who are interested in exploring beyond the status quo. You should return to your day job with a renewed sense of energy, and a plethora of ideas that will inspire you to do better, achieve more, think bigger.
For tech writers, that conference is Lavacon. It's dedicated to content strategy, which, in my mind, means thinking outside the box about why and how we produce content. Social media, new content types (such as mobile) and an increasingly global environment mean that the communication approaches we've used in the past are no longer good enough. The role of a tech writer is changing; there's simply too much content to have only one role dedicated to writing all of the docs needed.
My favorite session was Scott Abel's "Help 2.0" presentation. His ideas about how to leverage the crowd to create and organize content were revolutionary. I think that crowd-sourcing has great potential for developer docs and have been thinking about how to do it ever since.