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.
I personally didn't like it any better than L4D. There are quests to pick up and some RPG elements as you said, though. My point was, I didn't find it any "better" than L4D. It felt like playing a Co-op game without any friends, and because of that it got boring quickly much like L4D single player did.
I wish they would make an open world zombie survival horror game not based around multiplayer, but based on single player, with the structure of something like Fallout 3. That would be great.