While my take on the film isn't 100% positive, I found quite a bit more to like than dislike in it. As seeing the movie was my first step into the world of the Twilight Saga, and was what drew me to explore the Saga further, I'd have a hard time not giving it an overall positive review. Nevertheless, I'm going to start with what I found fault with in the film.
The big problem I have with the film, now that I've read the first two books, is that it has an underdeveloped, undernourished feel to it. As others have commented, the development of the Bella/Edward romance has just a bit of a rushed feel to it; a little more screen time spent on that might have paid large dividends. I also think that the relationship between Bella and Alice should have been developed more thoroughly; the Phoenix hotel scenes would have been the ideal spot for this. Alice's speech "do you think any of us want to look into [Edward's] eyes for the next hundred years" definitely belonged onscreen, and Jasper's "you are worth it" could have been worked in as well. I can also think of a few other ways in which very small additions--the deleted "she's brought him to life" scene between Carlisle and Esme, the other Cullens burning up the dance floor at the prom--could have enriched the narrative quite a bit. Any of these elements would have been much better use of running time than James playing with his food.
However, I found a great deal to like in Twilight. In order of importance, I'd point to:
1) Bella and Edward. I thought that both Kristen Steward and Robert Pattinson did very credible jobs. I know that not everyone is a fan of Stewart's Bella, but I thought she did very well at giving us a believable screen character, and that she and Pattinson had good chemistry onscreen. Pattinson excelled at bringing out Edward's "I am damned" persona which is so central to his character, at least in the first half of the saga.
2) The Cullens. Casting of the Cullen-Hale clan was always going to be problematic--it would probably have been impossible to come up with a cast that matched Stephanie Meyer's physical descriptions of the clan. Emmett and Rosalie, especially, would pose problems. The list of young, convincing actors who have the approximate dimensions of an NFL tight end is zero at this time, as far as I know; as for Rosalie, I can only think off the top of my head of one actress ever who could measure up to the picture of unearthly beauty that Meyer gives us (if you're curious, it's Grace Kelly).
That being the case, the casting department did well to find performers who could get the characters right. Ashley Greene, as I've mentioned in another thread, was outstanding as Alice. She clearly has the character nailed down, and she did a wonderful job of conveying it despite not having much dialogue. Peter Facinelli, although ill-served by the make-up department, is likewise spot-on in his portrayal of Carlisle. I know that Nikki Reed's casting as Rosalie is a bit controversial, but I thought she did a nice job--her glare at Bella in the baseball scene conveyed a lot in just a couple of seconds. And Kellan Lutz, while not quite the behemoth Emmett of the books, has a very solid physical presence that is appropriate to the character. Unfortunately, Elizabeth Reaser as Esme and Jackson Rathbone as Jasper were given little chance to shine; the two of them might have been ciphers, were it not for:
3) The Baseball Scene. Many people's favorite scene in the movie, this is a textbook illustration of why the First Commandment of Filmmaking is "Show, don't tell." In 2-3 minutes (prior to the nomads entering the scene), we not only get an innovative action sequence, but we learn a lot about the characters involved in the scene as well, and we get a real sense that the Cullens are a family.
4) The Opening Scenes in Forks. When I watched the film again tonight I was impressed with how these scenes gave the whole story a "this is happening in the real world" foundation. The "ordinary" population of Forks is never going to be the focal point of the story, with the partial exception of Charlie, but it's important to establish them as a believable backdrop to the story. Two performances, I thought, stood out--Billy Burke as Charlie, loving but a bit inarticulate at times, and Anna Kendrick as Jessica, nicely conveying the mix of friendship and rivalry she feels towards Bella.
Edit to add:
5) Carter Burwell's Score. Burwell is, for my money, one of the best film composers working right now; he's done some wonderful scores for the Coen Brothers. He provides another fine one here. There's lot's of music filled with ominous overtones, which is very appropriate, but he also lends a more lyrical mood when it's appropriate.
"May the hinges of friendship never rust, nor the wings of love lose a feather"--Scottish blessing