I don't have anything against The Witcher as a series. Well, there are a few things. It takes the whole male power fantasy thing to unbelievably juvenile levels, the writing completely lacks any subtlety in trying to deliver a message, and the combat in both games is mind-numbingly boring. Still, when it comes to specific complaints, what I have against The Witcher was probably fixed in The Witcher 2, and what I have against The Witcher 2 was just them messing up something that was done right in The Witcher.
To me, The Witcher (at least the Enhanced Edition) was actually a very engaging game. Yeah, the combat was boring once you realized the few potions that you needed to power through even the toughest of battles, but the game never seemed focused on combat and at least gave the sense of being a Witcher. I guess Chapter 2 was also a slog if you didn't allocate your traveling between Vizima and the swamp properly, but but it wasn't that bad. Overall, it had some interesting characters, a compelling (albeit preachy) story, and a fantastic world. It also had a charm that few dark fantasy games have.
The Witcher 2, though, was just...yeah. Personally, I'm baffled at how it managed to receive any critical success. I guess the lack of a "save the world" story was nice, but the writing and pacing around it was unbelievably horrible. The combat system looked better on paper, but it managed to be even more monotonous than the original game, and that takes a lot of boneheaded decisions to accomplish. The world wasn't that interesting both from a writing standpoint (which I've already mentioned) and from a design standpoint. It just always felt way too restrictive. The art style didn't really appeal to me, but that's sort of subjective. The graphics, to me at least, also felt off, like they were designed to draw attention to every little mistake in the textures and animations. I guess the voice acting improved (not that that was a hard to do), and there were still a couple interesting characters (i.e. Roche and Iorveth). Still, the game was just an absolute slog that I couldn't wait to put behind me so I could just wait for the sequel, which now I'm very unsure of because of how much I disliked The Witcher 2. All I can say is if there is a stealth section in the tutorial of The Witcher 3, I'm putting it down and never looking back. And to make matters worse, I haven't even begun to describe all the little details that made The Witcher 2 even more unbearable, but I could spend hours listing them all so I won't bother.
Overall, I don't mind The Witcher series. There are still some interesting plot threads that I'm looking forward to seeing resolved in The Witcher 3. I may even go back and play the original game on occasion, as I find it to be one of the most compelling fantasy RPGs I've ever played. But I think that the second game was awful, and I'm not sure if that's because CD Projekt didn't know where to go with the series or if they are just that bad (though it at least sounds like they've learned from their mistakes in that game). They clearly have passion for their work, but that doesn't always translate to a good result.