This is going to be really rambling.
tl;dr version: Witcher 3 is a great game, and I would unequivocally recommend it to anyone who loves RPGs. I don't think it innovates anything in the RPGs genre, though. Also I wish it were more open-world than it is.
I have mixed feelings about Witcher 3 that are at least in part caused by the hype surrounding the game. More than one person expressed to me days after the game launched that it was (in their opinions) the best game they'd ever played. Innovative, deep, replayable, great story, etc. I didn't expect to think it was the greatest game I'd ever played, but I guess I was expecting something more unique than it turned out to be. This is the only Witcher game I've played, and I'm only about 10-15 hours through this one; I'd like to leave open the possibility that the game will blow my tits off as I get deeper into it.
I'll start with what I liked. It's a great game. It really is. The setting is beautiful, the cutscenes are good, and the core mechanic is robust. I dunno how much detail I really want to go into because every time I think of an example, ten more pop into my head that I think deserve a mention. So maybe I'll just say that it's good at everything you want a RPG to be good at: there are lots of different character builds; the setting it beautiful and detailed; the fight mechanic is actually engaging; the characters, even the NPCs, are vibrant; it's massive.
Here's what I don't like about the game: I don't really think it innovates anything. I hesitate to call this a criticism because it certainly isn't required that a game reinvent a genre to be good. To me, though, the lack of novelty in the game keeps it from being as engaging as I was hoping it would be. Every element of the game is well-executed, but it's all been done before: protagonist goes from town to town finding jobs on message boards and expanding the map as XP allows. I guess it just feels like Witcher comes up a little short when I compare it to a series like TES that took the RPG genre and added something new to the underlying architecture.
This could also be because I don't give much of a shit about the story or characters. It's not that the story and characters are poorly done, just that I don't care about them at all. The Elder Scrolls suffer from the same problem for me; I really just wind up getting into all the side quests and never really focusing on the primary story. This is probably because I'm ultimately worn on out the high fantasy RPG. I'm over it. I'd kill for pretty much this exact same game in a scifi setting.
I was going to ramble some more, but you get the idea.