Well Twilight stopped my lurking ways and forced me to come out in the open. So, let's get started on this rant.
On the subject of whether it's acceptable to change established mythos, I would say it is perfectly fine to change them to better suit your ideas. However, Meyer went beyond that point. She completely and utterly destroyed the vampire folklore from it's very root. Let's take a closer look at the meaning of "Children of the Night", and what that represents. Usually, night is the time when strange things start happening. It is a well known artistic motif. At night, the supernatural takes center stage, uncertainty replaces the certainty, a completely different world shows itself, with rules that are fundamentally different from our own. Vampires are supernatural creatures belonging to this world and not being able to walk in the daylight pretty much emphasizes this aspect.
And Stephanie Meyer took all the mystery behind vampires and burned it entirely. I'm going to put a small spoiler warning here, because I will be referring to the last book in the series, "New Dawn". Vampires walk in daylight, they go to school, there is a half-assed scientific explanation to their origin, they have their own government (yay, politics!) and to top things off, Edward and Bella have a child together. To quote the fat guy on Two Stupid dogs: Ain't that cute...BUT IT'S WROOOONG!!! To bottom-line it, Meyer took all the interesting parts about vampires and turned them into a big, nasty wet-dream.