Many times, I'm a driven completionist... but in most of these cases, the game fits into a pretty short list of variables. Other times, the shear mass of available playstyles or customization leads to my falling off the wagon into full-fledged altoholism. With the desire to play multiple characters within a single game guiding my actions, I may never see the "end" or "end-game", in the case of MMOs or open-play RPG sandboxes.
Fallout 3 (and its many mods), led me to NEVER see the final battle, first-hand. I did, however, play a thieving sniper, a melée/explosives expert, a scientist/energy weapons specialist... the list goes on.
Another example is my most recent game, Mount & Blade: Warbands. I've had a Vaegir archer who specialized in manhunting, a Nordic thrower (3 stacks of throwing axes and a shield) who spent more time in jailbreaks (getting others out, not himself) than not, and I'm just starting a Scalian tactician who uses a sword and shield only when absolutely necessary. In all cases, I've not taken a side or sworn to a king or rebel usurper... just explored, taken odd jobs, and expanded my wealth, reputation, and my troops' experience and equipment.