With Dragon Age Origins and the Mass Effect series, BioWare did excellent in constructing the games such that they have an immense replay value.
Essentially, the replay factor of the game is achieved by forcing players to make choices that drastically alter the outcome of the game, but revealing the outcome only at a much later stage in game, too late for the player to make a 180 degree turn, load a saved game, and choose a different option. Also, instead of a direct and obvious correlation between action and consequence, the results cascades throughout and unveils itself in bits as the story progresses. In Mass Effect, I find myself thrown into situations whereby I have to choose between two set of lives in my party, killing off an entire character and the resulting story arc. This is another area where I have to applaud BioWare for – the audacity to kill off a party member entirely. They do it well too, and the death makes sense in the context of the situation, unlike how Miranda Keyes was killed abruptly for no good reason in Halo 3 after all her character development and progression. In Dragon Age, the opening of the story differs depending on the player’s choice of class and race.
Thus, to experience all the possible permutations of the game, the player is forced to play it multiple times. Rerolling in a single player RPG can be made fun too, if the correct formula is applied. This is an area where a lot of games fail, especially in Dungeon Siege 2, where the only thing to be gained from running through a game multiple times is to unlock and play on a higher difficulty level.