There are many things that will make this game 'ground breaking', and I only know a few of those reasons.
A sandbox game typically means that everything in the game is built by the players except where obsoletely required. I believe everything in Wurm is player built at some point, except for Goat Horns - those that play the game SHOULD know what that is :P
Prelude is when the game goes live. Players will build and construct what is needed. Built Tribal Totem Poles, and will of course insert quests into the game.
Unlike other games thou, you likely wont find quests to kill ten bears, or the like. However you may find them if that is required to advance the tribe.
To my knowledge, we don't know why quests are in the game, however it will be a fun exploration for us to make the quests, not to have quests forced on the player (from the game).