The scary thing about the slow-moving ones is the claustrophobic feeling the book generated, like that pilot lady who got dumped right into Z country and had to be careful about noise and find a safe spot to sleep but still got surrounded. It's a numbers game, the terror comes from being outnumbered, surrounded, and with no place to hide. If they really wanted to they could have made the slow thing work, it just needs a director who's good with atmosphere. It just feels like they wanted to take the lazy approach because it speeds up the narrative.
I thought the book did an excellent job describing in-depth what was happening before and during the war, or at least enough to make a movie with. General Raj Singh (the one you couldn't remember how to spell) showing up to detonate those charges and stop the last-resort nuking would have been an awesome scene. And frankly, it's not intended to be a linear narrative, it should have played like a documentary from the way the book was structured, so it would be expected to be less focused on a single area by the readers, and it could have been an original approach for those unfamiliar with the actual background. I think it would have been worth the effort to make it reflect the book more accurately. As it is, the trailer makes it look like a full-movie version of what's happening in 2004's Dawn of the Dead where it shows what's happening on that montage while Sarah Polley is unconscious.
True, they could have done it pseudo-documentary-style like the book, basically stringing together a series of narrated flashbacks rather than a more central linear plot, but it's pretty clear they wanted to cover the actual war which, while you can piece it together from the book, isn't really covered. I mean, you basically get to hear about one major engagement in several different countries and...that's it. What happened after
Yonkers? We're just suddenly "later, after everyone was dead, we came back and did it right." The whole "war" part is pretty much summarized in one skirmish and that's it. The bulk of World War Z is build-up, society's collapse and mop-up post-collapse; it's not really zombie-focused most of the time, despite that being the core drive of events. Zombies are just the background while Brooks goes through a series of "what if" scenarios for how various regions would respond to that level of catastrophe. Translated into a movie, this would most likely mean not much zombie screen time and a lot of political/interpersonal drama. Which, hey, don't get me wrong, that could make a great movie (I'm thinking something akin to Clear and Present Danger-type content...with zombies), but it's not really a "zombie movie" in the sense that 90% of people would be expecting to get out of their ticket to see a zombie movie, y'know?
Sidenote, why did I remember Raj Singh's name as being something four syllables long? Hm. Must be thinking of someone else.