@caruga
Here's a question for you, considering that "Open World" is a form of level design in video games.
When do we start measuring that a game is actually "Open World"?
Perhaps a game doesn't start as an "Open World" game for the first few hours, but later as you unlock areas it actually becomes so?
All "Open World" games
should have limiting factors otherwise short-cuts can be taken. For example in Fallout 3 you can travel nearly 100% freely everywhere and because of that you can skip huge portions of the plot if you know where to go--I don't mean to put words in the developers mouths here--but I don't think they want you skipping over hand-made content they've spent an incredible amount of effort creating. But just because there might be or are
some restrictions, doesn't make it not "Open World".
I think The Witcher 3
is an "Open World" game, despite some regions or very specific areas that we
might not be able to reach initially due to story progression reasons. From the looks of the gameplay that we've seen so far we're able to approach our objectives in different ways and get to areas in different ways which makes the experience "Open". Another, but more technical, reason a video-game might be "Open World", is just as you've said, there are no loading screens when moving from indoors to outdoors, the world is large and seamless, and loads the environment around us as we travel. (However loading-screens for fast travel make logical sense, they're used as a cover as the game literally teleports you from one place to another and loads all of the objects around you.)
My opinion is that "Multi-Region Open World" pretty much nails what The Witcher 3 is.
We have multiple huge regions available to us, with many points of interests and things we can do or see, and how we approach our objectives whether they're part of the main story or a side quest in terms of time, pathing, people we talk to, people we ignore, previous quests completed, and previous choices made, all while knowing we have different options available to us, are entirely up to the individual. That in my opinion defines an "Open World", and it's exactly what The Witcher 3 is.
TLDR; The Witcher 3 is a true open world game, as true as it gets for a game that's not procedurally generated all the way through. We are able to travel how we want, where we want, and have an array of choices on what to do, not to do, and say, or not to say. Small area limitations do not make a game not "Open World", they're there for specific reasons and are designed to not subtract from our experience.
Whew.