The thing I like most about good sandbox games - their necessarily incidental storyline - is also the biggest problem when it comes to their staying power. I'll happily spend hours screwing around, setting and chasing my own objectives, probably for longer than I'd play if the game was constantly telling me exactly what to do.
But that lack of a strong narrative prevents me from feeling like I need to go back to Far Cry 3. While I'm in the game, I'll always have time for one more hunting mission or radio tower or stronghold, but once I turn it off I don't have any compulsion to start it up again.
But I've often felt that a strong story comes at the cost of player freedom and vice versa; there's no way to consistently tell an emotionally rewarding story, or even to have an interesting player character, when every moment can be spent in a thousand different ways. Jason's supposed to be trying to rescue his friends and escape from this island, but as a player I haven't been given a good reason to care about them. He's supposed to be new to the combat, to hunting, maybe even to driving - but I've been trained to do all of these things by ten years or more of gaming experiences.
What I want, as a player, from Far Cry 3 is a long way from what the character Jason is supposed to want, at least at the early stage of the story I'm at. I've read enough interviews and deconstructions to know that, eventually, the allure of these extracurricular activities becomes a character point for him - violence as addiction - but the player's already there, looking forward to the next shootout, searching for the next objective, waiting for the next achievement.
Another problem with sandboxes is that, once you've unlocked one supply point, hunted a few animals and had a go on each vehicle type, you've basically seen everything. Sure, some of the later animals might take a few more bullets and the supply points on the southern island will probably require a slightly different strategy, but I've done those things already. Everything else from here out is a repeat of something I've already experienced.