http://www.pcgames.de/The-Witcher-3-PC-237266/News/Alles-zu-Quests-Minigames-und-Monsterjagden-1127210/
Dunno if German articles are to be mentioned here as well. This one is mainly about the quest structure in TW3 and features the insights of an interview with Konrad Tomaszkiewicz.
The quest structure consists of three layers: main story line, side quests and monster hunting. A main story line quest will take approximately three hours on average, side quests are "very extensive" as well.
The monster hunting has a slightly smaller scale - in general you'll help villagers with their monster problems. As soon as you know which monster you have to face you may track it down - but without decent preparation you might not survive the following encounter. Therefor it's advisable to gather information beforehand, e.g. by looking for books on the topic. If you accumulate enough knowledge about the monster in question you'll be able to "deactivate" certain monster abilities, which will drastically improve your chances. After you've slain the monster you can bargain for a higher reward.
There will also be several mini-games which feature associated quest lines, e.g. one for a collectible card game.
The last layer of the quest structure are "small encounters in the game's world", which feature a story be themselves too. Last but not least there's treasure hunting, where you'll find recipes and ingredients/resources for powerful weapons, armors, potions and the like. These can gathered from monsters, on he seabed and so on.
Sorry for the somewhat awkward translation, I just did it on the fly since I'm kinda in a hurry.