The problems are threefold:
~the hero, Edward Cullen, is perfect
In some contexts, this might be an overstatement. In the world of Twilight, it is an understatement. Your typical romance novel hero needs flaws: a scar from a gun fight years ago which gives him a rugged charm; an emotional wound dating from his mother's death; a weakness to Kryptonite. Typically he has all three, making him all the more attractive, something to "fix", am I right, ladies? Edward Cullen has no flaws. He's model gorgeous, which the narrator (cypher for Ms Meyer?) never ceases to let us forget. He's smart. He's funny. He's caring. He's always right. And he's boring. The supposed flaw that his skin glitters in the sunlight not only doesn't make him interesting, it makes him kind of campy. Even the imminent danger he poses to the heroine because of his vampiric nature doesn't save him from dullness.
~the heroine, Bella Swan, has no personality
The book begins promisingly enough, suggesting that Bella is a sarcastic, vaguely artsy introvert. She is clearly clever and willing to strike out on her own, moving to the overcast and provincial town where her father lives. "Excellent," the reader thinks, "An acerbic, independent woman, crippled by her shyness but with a core of moxy." Absolutely not. Bella faints at the sight of blood, gets nauseous at the drop of a hat, and immediately succumbs to Edward's overbearing statements that he can't get too worked up around her. Thus, not only is he boring and she wimpy, they don't even have real chemistry. All the characterization exists in phrases like "'What do you mean?' he challenged." or "'I like that dress,' I opined." If the reader has to be told that Bella is being sarcastic, she's not.
~there is no real conflict
Meyer's protestations of Bella's blood pounding and Edward's longing gazes to the contrary, they never really get it on. To be sure, it's pleasant not to read smutty bits in a teen novel, but the plot is so chaste as to make me question Meyer's intent. Is this, or is this not a romance novel? When Edward has managed to secret himself in Bella's bedroom one evening and they've been cuddling and canoodling for a bit, he asks, "What do you want to do?" She, still breathless from his perfect presence in her humble room, considers and says, "I don't know." You don't know? Really? The "action" sequences are few and peppered between large swaths of Edward and Bella sitting around and talking. The villains of the novel are either easily avoided or easily defeated. Even the early uncertainty between Edward and Bella, the part where they either hate one another or misunderstand each other's actions has a certain inevitability about it. What is there to overcome?