Regarding I Am Legend, I liked it better when it was called The Last Man on Earth and starred Vincent Price.
...gods, that sounds douchey. Really though, they're both adaptations of the same book, and Price's version does it much better. Also I believe they were supposed to actually be vampires, not zombies at all, but the Will Smith movie doesn't do a good job of conveying that. On topic, I think the fast zombie trend is mostly prevalent because filmmakers figure audiences won't find anything slow moving to be all that scary, and they're often right. Not to say it can't be pulled off effectively, it's just that it's rather easy to screw up and fast zombies are easier to make scary.