To people who claim that GTA/Assassin are sandbox, I have to disagree there. Those are open world non-linear theme parks. Yes, you can do some "side stuff" in any order you want to, but in the end, you still have to progress through the "main stuff" to unlock more "side stuff".
Sandbox usually means you can create the content using the tools in game (not talking about SDKs, those are not part of game). For example, Spore was/is a sandbox, Minecraft is a sandbox, and so on.
GTA is more along what I was talking about earlier in the thread. It's a sandpark, not a sandbox. That's why it's massively successful, it caters to people on both sides of the spectrum.
For those sandbox types who just want to run around killing people, shooting cops, making money, or doing car tricks it's great.
For those in the middle ( the majority when it comes to mmos and even a large part of those who play only singleplayer games I think ) who like that stuff but also want some direction in where their experience takes them, the campaign adds to that.
And for those who simply want a good themepark ride from A to B without messing with the "frivolities" , they're covered too.
For instance, take the campaign story out of GTA and Rockstar would be like a CCP or Egosoft. One of those "niche" developers that enjoy a small but dedicated group of fans enjoying their games, but avoided by the majority of the gaming population at large.
The Elder Scrolls games are also sandparks, not sandboxes as many erroneously refer to them as. And we see how successful those games are ( the singleplayer games anyway, the mmo appears to be the upcoming black mark against the series from what I've seen so far ).