Zen Faulkes has a great post on the Better Posters blog, it's well worth a read if you think you will ever need to make an academic poster. This section nails it for me (emphasis mine):
Goals of a poster
Posters should get conference attendees talk to the presenter. Because attendees are busy, posters must grab attention, even if a potential reader is quite a long way from the poster. Similarly, posters should make an implicit promise to the reader that the gist of the poster can be grasped quickly.
Posters should also contain enough information that a person is able to read it and understand the main message.