There's no time limit. No restraints, other than the occasional Insurmountable Waist-Height Fence. No objectives, no requirements. You can do whatever you want, whenever you want.
Well, okay, there are goals if you actually want to, you know, finish the game, but why bother when there's so much to do beforehand?
Modern games increasingly lean towards having a central goal and an incredible amount [...] [tvtropes.org]