This way everyone that doesn't have that much time to explore all (like myself) can at least get a feel of quest progression and how it's working.
DoS is currently using a flag / trigger system, and is akin to Ultima VII / Serpent's Gate in this manner. Balder's Gate also comes to mind, as does Dragon Age.
However, DoS is unusual in that it is (attempting: there's bugs / lack of trees) focusing on multiple ways to satisfy at least the main quest trees; the sub-quests are entirely linear and shallow atm.
Hope that helps.
