For me it was the whole "subversion" of fantasy tropes to bring it to the standard of these recent grimdark tv shows.
Game of Thrones (TV show) — S1, Ep1 17 Apr. 2011 "Winter Is Coming"
Dragon Age: Origins — 3 November 2009
Wiedzmin I — 24 October 2007
I'd say they wanted to make their own Witcher, maybe not initially, but after CD Project success. And by them I mean not only Gaider, but maybe some of the producers. They also were ripping off Martin's books somewhat, but they couldn't have predicted the success of the TV show which came out two years after DAO release.
DAO's plot was way more traditional than both GRRM ASoiF and Sapkowski's Wiedzmin anyway. You got your standard Bioware Evil™ endangering the land, the whole Bioware cliche table with only superficial grimdark and political elements (mostly only in dorf noble origin story) added.