I think like someone said above it's all about how good the story is and if there is *character progression (new weapons &new locations that have real variety, new abilities that change the way the game is played, like speeding up travel in Infamous) and stuff like that that keeps a Sandbox game feeling fresh. It's true that its harder to set up big set pieces in the story in case you skip an area entirely. In general I find sandbox games that give too much freedom and don't hold your hand and guide you through the story have generally weaker stories. But of course they can still be a lot of fun.
Try a playing one of my favorite sandbox games: Fallout 3. Being able to play as good or evil make the choices you face in the game very interesting. For me it was thrilling to explore the wasteland and discover new locations on your own. Fallout 3 was definitely one of my favorite games of 2008.
I would also argue that ANY game gets boring after a while. Play any game too much and that will happen. For you sandbox games just get boring faster due to repetition and maybe the worlds are too big to travel?
Far Cry 2 had the too big to travel feeling. Driving around in the car constantly and stopping to repair was boring. Not too mention when I cleared out an enemy camp they would all respawn when I drove back a minute later. So annoying. I gave up fast on that game.
