The Witcher was mature? How?
The world lacked the fairy tale black and white morality and the motivations of individuals in it were wholly believable. The PC's actions had real consequences, and there was no "good" or "evil" path through the game, merely the choice between bad options. The game didn't care for mainstream opinions, and included sex, swearing, drugs, alchohol, rape, incest and all sorts of things politically correct games avoid. At the same time it didn't preach or take itself too seriously, knowing to joke at its own expense.
This kind of not pompous storytelling, unburdened with moral absolutes is probably only seen in fallout 1 and 2, and ultimately that's what made it popular and admired in the community. While the reviewers were essentially bashing the game for not paying them the usual bribe, the gathered grades of users on various game sites showed how people knew to appreciate not being treated as morons. As is prevalent in games today.
I still feel the writers of the game didn't really take advantage of the "mature" setting. They handled it like amateurs. Bad swearing, sex and slavery doesn't make it more serious, or tasty for adult gamers. Overall I feel TW was a half-hearted attempt to compete with Bioware etc. And don't get me started about the bad writing and the pale characters, or the overall setting, which didn't make a lot of sense to me. Not to mention the so called "moral" choices which were basically a joke.
While Bioware isn't that much better, they deliver at least a cohesive experience. They understand at least the basics of storytelling.