It's not for nothing that the worgen and the forsaken are fighting in game: it's effectively an objective correlative between the fictional setting and their individual roles within it and in their factions. Forsaken are cursed being trapped between life and death, while worgen are cursed being pinioned between beast and man. Each has to adjust to a new way of looking at existence and what his or her place in it is.

Each is their faction's 'monster', effectively: in a world of orcs and tauren, forsaken stand out as abominations rejected by the natural world those groups so revere (besides Sylvanas' former status as one of the high elves greatest heroes, this is how the forsaken and the blood elves manage to find common ground, as the 'other guys' from the supposedly shamanistic orcs, trolls and tauren) while the worgen are now the 'brutes' of the Alliance, with a once proud civilization left in tatters due to war and the imposition of a new way of life, which gives them their rapport with the night elves.