What has its impact on pop culture been? It is a good story, and it was adapted into a good tv show, but at its heart it is about zombies in the same way umpteen other books/movies and even comics have been about zombies. The only difference is that WD looks at the long term story of survivors, but aside from that how has it changed a genre? I like WD, don't get me wrong, but I can't see how it is still going to be relevant enough in 10 years to command even $500 for #1 when the show/comic series is done and folks have moved on.
I think it's impact on pop culture stems from its mass appeal. Sure, there have been plenty of zombie movies, stories, etc. But for some reason, this one seems to resonate with people. I can't tell you how many times I've overheard soccer moms having conversations about the show, and I'm willing to bet they don't read comics, or even know the show is based on one. Other than the original Night of the Living Dead, and maybe Dawn of the Dead, I can't think of any zombie-related media that achieved anything higher than cult status. As I type this, somewhere in the world, someone's grandfather is wearing a Daryl Dixon t-shirt! And I think that's pretty cool!
As for changing the genre, I don't think it did. The only thing that sets it apart is exactly what you said, it takes a long term look at people surviving a zombie apocalypse. Let's face it, there's only three things you can do with a rotting, shambling corpse; kill it, be eaten by it, or have sex with it. And only 2 of those things make for good tv (nobody likes to be eaten)