Small note: Just adding Sidequests does not increase replayability. You also need to make sure that different quests are available to the player depending on his previous choices. AFAIK, such decisions were few in DD, and with minimal impact.
I do think that replayability is overrated. In the modern gameworld, there are several high-quality titles that I may want to play at any given time. And my spare time does NOT tend to get longer...
So I actually like it sometimes when I know that I do not have to play a game several times to see everything (and I do like to see everything in a game).
I´m not saying replayability is bad, quite the opposite. I do say that not every game needs it. RPGs can benifit greatly from it, but only if the players actions can change the course of the game, and the PCs development significantly. (e.g. Planescape: Torment)
