Dead Island is a co-op game. It's not really any better than L4D if you're playing single player.
If you have friends to co-op it, it's a lot of fun, otherwise, not really a great game.
Weellllllll, I'd say it's better than L4D (or L4D2) in terms of a single player experience. DI has loot, levelling, dialog, stores, quests, and other RPG-type stuff that the L4D games lack, and they beef up the SP experience. I feel the individual combat is more robust and satisfying than L4D's. That said, it's painfully obvious that DI was meant to be played with others (to the point of immersion-breakage, like when a cut scene pops up and suddenly you're kicking it with three allies you've never even seen before), and you're definitely right that multiplayer (at least, if you aren't a victim of the many bugs) is the way to go to enjoy what the developers intended. Still, I did my first playthrough solo just to soak up the story, and it was fun enough.
Bonnar's right about the trailer, but that was something between a movie short and a music video. Hard to make a game live up to what amounts to an emotional kick in the balls.
Everybody good, plenty of slaves for my robot colony?