While it isn't as huge and easily accessible as Saints Row, it is a world that very much feels open and inviting of exploration. It's also not too open, so that you don't get lost easily. The style of open world in Dead Island is useful because it makes the game feel more hazardous, since you are always surrounded by zombies to one degree or another. Every turn is a risk, and backtracking even in teams can be intimidating. The game is very much an “on foot” open world game, which from a co-op angle is very good. I loved Crackdown and Saints Row's co-op, but in both games it's really easy to get separated from your friends. In Dead Island, the world is open enough to explore, but not open enough to get lost.
Because of this “open but with limits” approach, you are very much forced to stay together, like in Left 4 Dead, which I view as the ideal co-op game because of the fact that you have to stick together to survive. The additional players are inherently useful because taking on a large world full of zombies at every turn is never an easy affair, and your solo strategy is simply to survive. Teammates make the affair easier to deal with, but like Left 4 Dead, you never feel completely safe or overpowered. Plus, it’s always fun to kill zombies in plenty of beautiful locales that reward exploration with more sidequests and weapons. I've always thought that open worlds and co-op are a match made in heaven, but Dead Island somehow managed to make an even greater match when it threw in a zombie apocalypse.
The past several years have been great to open-world co-op fans. With the promise of Borderlands 2, Firefall and maybe a new Dead Island game (Dead World has been trademarked by the devs), the future is bright for open world co-op games. So, go out there into the big, shiny world and bring along a friend or two. Also, if a game wasn't mentioned in the list, please note it in the comments.