Really easy to sum up:

Witcher 3 is an RPG with a fixed playable character which does not let you move away too far from his basic personality. He is a grunty, old badass. You can choose wether he's more hateful or more laidback but not much more. This allows for an amazing storyline and I absolutely love the story of Witcher 3. For me personally, it's the best story in a video game I ever played.

Many other RPGs will let you choose your character completly unbound to any other background. Take Skyrim for example; literally the only backstory you have is that you were caught crossing the border between Cyrodiil and Skyrim. This allows for a heavy focus on roleplaying elements since you can choose everything about your character. I find it a bit nitpicky to argue that you can't finish every quest in a bad-guy manner which seems to upset some people. Apart from that though, you character is completly your own. The storyline can't be lined down to one specific guy however which makes it a tad weaker sadly. Skyrim is also one of my favourite games if not No. 1 but especially the main story isn't as great as Witcher's (for me that is!)

I also want to add that I find games that leave the choice of character open to you much more replayable than those which have a set character. I played Witcher 3 three times because the story is so cool but I got about twenty different characters in Skyrim with time invested varying from a few hours to more than one hundred.