I can understand "losing yourself" in the game for, what, maybe an hour? As long as it takes to ride around the land and see all the sights. Wonderful. But now what? With the exception of the scripted main plot missions and a (very few) scripted sidequests, the world is dead. The extra activities and challenges are painfully dull, and the opportunities for emergent gameplay are practically nil. In similar open games like GTA or Just Cause 2, there is a ton of random stuff to do. Parachute off giant buildings, ride on top of a plane as you crash it into a gas station, jump a motorcycle clean over a river, etc. You can entertain yourself for ages without ever touching a mission.

Red Dead Redemption. . .well, the game is a sandbox without any toys. What can you do on your horse? What can you do with the environment? What can you do with the inhabitants of the world? The answer to all of those questions is not "nothing," but it's pretty close.