A few reviewers noted Meyer's changes to the vampire mythos and why these changes don't work:
Remove those barriers and you remove the romantic tension. The great romantic stories, like Romeo and Juliette, are powered by social barriers to love. I don't see the barriers here. This is more like a made-for-TV teen soap opera where there is so little at stake (pardon the pun) that the big romance amounts to puppy love. (Lariat Movie Scope)
These aren't the only problems with Twilight's vampirism.
For starters, these vampires are almost as powerful as Superman. If most of them are evil, how do the good ones keep them in check? Why haven't the evil vampires taken over the world? They'd be almost unstoppable anywhere they'd go. A handful of them could walk into the White House or a military installation and take control.
Since they're invincible, why do they even hide? They could easily take over a city or town. If they offered immortality to anyone who obeyed them, humans would quickly become their faithful subjects.
The vampires wouldn't even have to convert anyone into vampires. Just the promise of immortality would convince many people to join them. Imagine how many political, corporate, and military leaders would prostrate themselves for a chance to live forever.
The movie's action
Some reviewers complained about the special effects. I thought they were okay. But the action sequences don't make a whole lot of sense.
Take the baseball game, for instance. If one vampire pitches with super-strength and speed and the other bats with super-strength and speed, it should be a wash. The pitcher should still be able to get the batter out roughly 75% of the time. But in Twilight, every pitch leads to a booming hit.
When Rosalie hits a ball a quarter mile into the trees, Edward chases it down before she can hustle around the bases. In other words, he runs a quarter mile before she can run 360 ft. Either Rosalie didn't get the power of super-speed or the filmmakers made a mistake.
Fleeing from James the tracker makes even less sense. When you're being pursued by the world's greatest hunter, is it wise to split your forces? How about if the Cullens hole up in their house, surround Bella, and dare James to make his move? What's he going to do...attack them one on seven?
For more on the subject, see Quileute Werewolves in Twilight.