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.
I would love to play that game. And I actually liked the L4D games quite a lot more than DI, in fact I liked it so much I organized a 16-man tourney for L4D2
But L4D with computer teammates was definitely a pretty barren experience.
Everybody good, plenty of slaves for my robot colony?