I think the reason why Witcher seems like it gives off a more adult vibe is because the "magic and fantasy" bit is not as emphasized. Not that magical creatures and fantasy elements are a bad thing, it's just that to a regular non-gaming (or maybe even to the casual gamer) person, magic fireballs and dragons are always associated to younger people, cartoons, and all that stuff.
It's just that the Witcher seems like it focuses more on realistic problems like political conspiracy, regicide, and "regular people" war, rather than demon invasions, magic, and dragons. But of course, the Witcher doesn't leave that out completely.
This is total, total, utter, hugely, bullscat, I seriously cannot believe anyone who played the Witcher series could make this kind of claim. In The Witcher 1, the first act had you saving/condemning a WITCH. The second act was the only good act in the game (and the only really good one in the Witcher series), it didn't have too much to do with magic. The third act involved utterly laughable time travel, and you somehow teleporting to this winter plain out of the middle of nowhere. Also, the Wild Hunt, the GHOSTS who ride all over the universe looking for gosh knows what.
That's not even to mention how 75% of the enemies in the Witcher are wild, absurd, truly stupid fantasy creatures, while in DA it's "bear" or "wolf" or "bandit."
let's move on to TW2. The first act, that had Shila de Tanserville (sp?), a sorceress (which is some kind of society in the Witcher world) with questionable practices. That was about it for main stuff. The second act, at least the Roche side (I have yet to play the Iorveth side), COMPETELY centered around these ghosts haunting a battlefield, and undoing a CURSE placed by a sorceress. The third act...the third act is a mess I can't really understand, outside of the dragon who's also a person. And what's his name, the kingslayer.
Edit: And TW2 focuses on the Wild Hunt, the "Spectral Horde," as well. And I suspect you know--they're naming the third game after their merry band of GHOSTS roaming the UNIVERSE. Very realistic, that.
All in all, the fantasy in the Witcher is absurdly greater than DA, and even greater than TES in my opinion.
Edit: And since we're here, sex in The Witcher is unnecessary about 75% of the time. The only time it has any purpose at all is to show that Geralt and Trish are with one another (the beginning of TW2, which does a nice job of flipping off the people who picked the elf in TW1) and Adda. Excellent, EXCELLENT job with Adda in my opinion (when the monster necklace vibrates).
Outside of that, though, it adds no story significance, does not aid character development, and does not help to define your character. It's completely unnecessary.