Players will earn ranks in WvW by earning World XP, which is awarded for doing pretty much whatever your character would normally do in WvW. Each new rank provides points to spend on abilities, all of which will start off as passive improvements like the ability to carry more supplies or do more siege damage. In the future, the developers want to add more active options so that WvW players feel as if they have an alternate advancement system past level 80, giving just a little more motivation to hit the WvW maps all over again.
[Thanks to Pete for the tip!]
[Update: Just as this post hit the front page, ArenaNet published a dev blog by Angel McCoy that focuses on the living story. Not only does McCoy explain how the paradoxes of the living story will work themselves out, but she teases that ArenaNet has "increased the amount of content (it will) be delivering, starting at the end of March." Good deal!]