The Witcher III is much more open than Witcher 2 - while there are indeed main quests (not just one), they do branch off and there are hundreds (literally) of optional side-quests including treasure hunts, raids and fetch quests. These are all optional, but they are generally so enjoyable you just want to do more.
There are no chapters and you can always go back to the main story - you can't fail this, but you can fail some quests, but even then, you are probably only losing a bonus item or card (see more about this later, in the form of Gwent) or money really.
As expected, there is repetition to a degree (especially the treasure hunts out at sea), but even so, you are likely to be exploring new areas, finding new things and gaining experience and items along the way. You don't have to follow the main quest at all, it'll just always be waiting for you. Just going out looking for monsters (of which there are a huge variety from small to pretty big) is fun in itself and you never know when you're going to run into a troll, cyclops or giant spider etc.
The story is also very well written and acted, with a huge amount of dark humour throughout. There are several sex scenes in the game, so if you want to play it, make sure your Mum isn't visiting/around - they pop up a number of times. Some are optional though.
The world itself is also bustling and very interesting, even in quiet moments - watching the grass, bushes and trees blow as the sun sets and rain picks up is a wonder to behold and really makes the world feel very real.
Be aware - there is a fair bit of Windwaker style sailing between lands (sometimes) which could be seen as boring to some, but it's pretty optional as most placces can be quick travelled to once road-signs are located.
Honestly I can't sell this game enough. It won Best Game Of The Year awards across the boards and deservedly so.
Then there's a mini card game called Gwent - collecting all those cards is an achievement in itself and the game really is excellent - very deep, with only a few very simple rules.