While I also expect further acts will be made available at full release, I am hoping they add in or flesh out further side quests and locales, perhaps including additional race/class type quests to test various facets of other races and classes as they are introduced into EA. Maybe even some higher level quests that will be transplanted to other areas/acts in the final release, but allow testing of higher level skills/spells. At some point the coding for higher level stuff will need at least some degree of play testing, or Post Final Launch will still need a huge amount of patch/re-write/tweak after the game is in full release.
