From what I understand you can be doing a quest and then randomly decide to go explore some random cave instead, and you get rewarded for exploring. Not sure if you will be able to get equal rewards as you would from raids, but you do get rewards for just walking around even if you don't do quests.
Also who cares if WoW players come and then leave? I don't know why everyone seems to think if a game isn't a WoW killer then it failed. As long as the game keeps enough players and gets enough new content to keep me interested, and they don't create a pay to win cash shop, I don't care if any WoW players give up WoW for ESO.