I've read a handful of reviews, here's my 5c on some of the hotter topics.
RE Main Quest: Few of the reviews downplay it, some outright praise Geralt's personal journey. That's something to remember, maybe for new Witcher fans. An overwhelming majority of RPGs place the protagonist in the middle as the chosen one/god/hero/lord of everything. The series isn't like that. Sweeping changes happen, but often as a backdrop or around your character rather than to or purely because of your character. Keep that in mind you'll always, in a way, play second fiddle to the world itself.
RE Endings: Reviews are divisive but remember there are multiple endings with variations OF those endings based on progress of the plot. It seems you just kinda get a summary of how it all ended, for better or worse. You're never guaranteed a happy ending. Or a cathartic abundance of closure. Maybe your ending will be exactly as you wanted. Or maybe it will be unexpectedly tragic and a bit of a downer. That's how these games roll. The Witcher 2 fucking ended on sorcerer/sorceress genocide and a mass invasion, neither of which you could prevent.
RE Fetch Quests: Seems the main quests are the culprit, and I could see how: you're probably tasks with far reaching "get X of Y" to progress. But most reviews don't paint fetch quests in the same way people are familiar with from the likes of Inquisition and Skyrim. Most reviews praise context and development of quests. Most quests in games can be distilled down to basics of fetch, but context is key. And that context, along with ramifications, sound like the game's biggest strength.