I guess I just find a lot of the trappings boring. Conversation trees, juggling a large inventory of items, trading, crafting, levelling up... It all feels like stuff that gets in the way of a game for me, rather than being stuff that's enjoyable in its own right. Managing it all isn't a satisfying challenge to me, and doesn't give me a great sense of depth either, it just feels like a hassle.
Some of those elements have certainly found their way into other games that I've enjoyed (GTA, Doom, The Last of Us etc.) but they're usually pretty basic and you can more or less ignore them for the most part without it affecting the game too much.
I'm not 100% set against RPGs in general or those individual elements specifically, it just doesn't tend to be stuff that gets me excited about a game.
Anyway, I'm staying open-minded about Witcher 3 so far as there's a fair amount to enjoy: it looks beautiful, the story is relatively interesting (at this early stage) and the combat system seems like it'll be pretty decent once I've mastered it (and maybe reassigned some of the controls!).