I'm looking forward to the Buzz Lightyear ride. I was last there in 2004 so that's really the only new thing (other than the parades/shows). If you're not going til 2008 then you'll get to experience all the new stuff coming soon in WDS. Tower of Terror (I think it'll be ready then), Cars, Crush's Coaster.
Since 1997 loads have been added/refreshed. Space Mountain 2. Indiana Jones (I think). I think you'll definitely need a lot longer to see everything than last time you went.