Its success is just another nail in the coffin of single player RPG games. Not that they will cease to exist, but we could have done without Biowares SW MMO and played KOTOR 3 or something of that sort.
What is it then?
MMO's cause some nails, but what really causes it is, people not buying them. The Witcher a game you are so fond of sold what, 1 million. I don't think NWN2 and its expansions did that well either, neither did Mass Effect. Why I don't know, for whatever reason (some peole with very big heads will chip in here about quality) RPGs/FPS don't sell that well on PC anymore. You watch DA:O, AP and Risen will all sell better on the consoles despite being better on PC.
I think a possible reason for PC games not selling well, is because all of us PC gamers are on forums posting, instead of going out and buying games.