Twilight wasn't as bad as I expected it to be. It's not really any worse than the average fanfic. That said, it does have some problems.
First, it is too long. The story doesn't pick up until halfway through the book, by which time you've read over two hundred pages about the very dull Bella Swan. Now, I don't require constant action, and I'd be fairly happy to read a couple of hundred pages developing the characters and their relationships, but that brings me to the second problem...
The characters are poorly developed. Except for a few very basic things, we just don't know anything about them. Even Bella, who is telling the story, is pretty much a mystery (if I'm being charitable--uncharitably, she's just empty). I'd expect, after reading 500 pages from Bella's point of view, to have a good idea of what she thinks and what she'd do in any given situation. But, except for a few specifics ("Bella, who are your favorite baseball team?" "The Forks Vampires!" "Bella, want to go to a dance?" "Only if I get to dance with a vampire!"), I just don't have a good enough grasp on her to make any predictions.
The first two problems tie together, too. The first half of the book or so is spent developing (poorly) Bella's relationship with her schoolmates and father, and the second half of the book is spent developing (poorly) Bella's relationship with the Cullens. But the Cullens don't really figure into the first half, except for Bella mooning over Edward, and the other residents of Forks don't really figure into the second half. I think that Jessica and the others get mentioned for about one line after page 300, right at the end.
My suggestion to improve the book: spend fifty or seventy pages introducing and developing the characters that matter (pretty much just Bella and the Cullens, with a special guest appearance by Jacob Black, I guess), and then dive into the story from page 300 or so. That way, you get a nice, 250 page novel that you can easily read in a couple of hours, and your time won't be wasted by characters getting abandoned halfway through.
I hate to write a review that focuses only on the negative, so let's look at the positives, too.
Once the Cullens showed up (not in the distance, at the beginning, but later, when Bella is properly introduced to them), the book really did pick up. Not just because of the action, but because Bella started meaningfully interacting with other characters. Pining over Edward and ignoring Jessica isn't meaningful interaction. But Bella becoming friends with Alice is meaningful. There's not nearly enough attention given to it even to call her friendship with Alice a minor subplot, but at least we see that there is more to Bella than just Edward.
The subplot with Billy Black trying to warn her away from the Cullens is somewhat interesting, as he does have the best of intentions, and he's in the know, so he and Bella are maintaining the masquerade during their discussions, including those that occur by proxy.
Really, I think that whenever Bella is paying attention to someone other than Edward, the book improves. The main problem I have with Twilight as a romance is that the relationship between Bella and Edward is just obsession with no basis (unless "you smell so good that I want to kill you" and "you're so pretty that I want you to kill me" counts). Outside of the main couple, though, Meyer spends a (very) little more effort developing things, and the book is better for it.
Well, that turned into a long, rambling review, and even my positives section was filled with negatives, but I wanted to get my thoughts 'on paper' while they were still fresh.
Recommendation for this book: don't bother.