One of the most overrated, institutionally over-hyped, disappointing examples of mediocrity I've played in 28 years of gaming.
I'll hop right to it, the way I hopped right from finishing Witcher 2 to playing Witcher 3.
As far as combat is concerned, it's still intensely dull and only gets easier with time, with the only improvement over Witcher 1 and 2 being free DLC to replace animations. Maybe you can call a separate dodge key aside from roll spamming an improvement since Assassins of Kings. Other than that, you'll get the same moves and the same mashing of the dodge-roll and counter attack/parry buttons.
The game world is packed with meaningless "points of interest" that more often than not don't even have quest relevance, or start a quest, like the tens of dozens of hags guarding merchant-fodder chests all over Velen. On the rare occasion you stumble upon an area that does have a story, it usually ends up being a tease and the quest ends just when it was starting to seem cool. Really the only map areas of slight consequence are those that used to be villages and have been overtaken by bandits or monsters. In which case you might luck out and a half-crappy herbalist merchant will live there in the future. Even the villages are much the same way, and there are SO many villages that most of them have no personality or unique attributes. You will never go to most of them again, either, after doing their quests. Oxenfurt, Novigrad, White Orchard, Kaer Trolde, Kaer Moerhen, Beauclair - these are the places you'll spend the majority of your time, other than running in and out of Crow's Perch all the time to use the master armorer.
The quests are boring. The quest chains in the vanilla game, parts of the story that are worth the time spent writing them, are few and far between and they almost all involve direct continuations of individual characters plots from Witcher 2 - Triss, Keira, Dandelion, Zoltan, Roche, Radovid, Sile, etc. Everything else amounts eventually to a rinse repeat of "Go kill this variation of wraith or vampire or dagron or thing."; "Go hold your witcher sense button for 7 years and run in generally a straight line until you've followed an annoying scent or trail of bootprints" or "Use this inorganic item to do a thing" and then come talk to a person to get 5 orens and 12 xp and regret ever caring. It doesn't add depth to the world, in a large chunk of scenarios.
Music is repetitive ad nauseam. The exact same music plays in the exact same situations all the time and it's not even as charming as when Final Fantasy did it. Fortunately, there are a few respites such as "Berries and Lilac Sweet". There are only so many times you can hear "na na na na na na na nehnehnehnehneh nah nah nah nah nah nah nah nehnehnaynehneh" before you no longer even want to attack anything. To that end, every single song in this game will get stuck in your head for eternity. It's not that the music is bad - it's great - but there's no variety.
The story itself receives way too much praise for being slightly above average. It has been propped up by 12 year olds for being more mature than your standard call of duty, and this almost entirely comes from a cliched over-reliance on portraying all characters as filthy, rude, disrespectful, duplicitous, officious, pompous, paranoid, asinine, vulgar, stupid, or any combination thereof. Protip: Trying to emulate George RR Martin by making people get raped and murdered and tortured at every possible turn does not give your story the same quality. On top of this, for a Polish game - Witcher 3 is utterly covered in British and American references ranging from Monty Python to Quentin Tarintino. At times it comes off as hilarious, others it's just stupid.
On that note, don't even get me started on the asinine FF13-like recap of the story every single time you have to load a fresh save or travel between countries. Dandelion never shuts the **** up, and it makes you wish you could kill him. There's no setting to turn this off - I checked. Just a mod aptly titled "Shut the f*ck up!". Sorry console bros, you don't get that option.
My biggest problem with this game is that Hearts of Stone by itself was single-handedly better - in all regards - than the 200 hours I spent on the base game. And the sad thing is, Hearts of Stone didn't even add much to the game - just enchanting and Ofieri gear and literally 4 or 5 long quest chains that were all tied together. But those quests were great. The same can be said about Blood and Wine.
Overall the game is a decent RPG simply for the fact that it has a ton of content, but have no illusions: 75% of the content is brutally boring and copy pasted. I've never been so happy to finally get to the end of what became such a painfully tedious game. The expansions were the high point of this game and you can't help but wonder how they did such a good job with the expansions, but such a mind-numbingly boring job with the actual game.