What do you mean by how open exactly? In typical MMO's your free to go where ever you want, and do what you wish except for a few exceptions: level restricted and quest restricted.
Level restricted means that the enemies in an area are much higher level and expect to die very easily. It acts as a barrier. By quest restricted, the area doesn't open up until a certain story quest is done/completed.
ESO has both, but you only really run into the level one more due to how the world is split up into zones. You are dropped off into an island or a territory and can go where you please as long as the enemies don't bother you. Once you complete the main quest long enough, a new area opens up and can explore again.
My nightblade does just as my skyrim character did, walk around and explore while looking for alchemic ingredients for potions.