@Vim: Funny, I came here through google search of "witcher ending disappointing"
Imo DA:O's ending was much more immersive. On one hand, I was really impressed by the choice I had to make in the end of DA, on the other, the possibility to talk to the people from my party after the game was much more impressive, then a single phrase by a side-character of the Witcher. Overall, the DA's ending left me with feeling, that something actually went on after the game was finished...
On the other hand, I must confess, that playing the actual game I had more fun with Witcher. Some of the (already mentioned) reasons would be the fact that the world is much more immersive (where the only colour in DA's world would come from the companions you run with in DA, the colour in Witcher would be all around); and the fact that combat was much more fun in the Witcher . Then, again, romances in the Witcher were a no-brainer mostly...
As a side note, I have to say that the world, most of the (often lovable) details, atmosphere and lots (I mean LOTS) of quotes were actually very faithfully copied from the book (the one I know quite well and like a lot) by CD Projekt. For me the problems with this approach were the fact that the scenario was quite predictable and some choices felt as if these were already made for you by the book. It also means, that the scenario (including its consistency) was much easier for CD Projekt, than for Bioware, so comparing them directly is not that honest.