There are some interesting things to be found here, notably a couple of fine supporting performances, but the movie is done in by some lazy screenwriting choices and a horrible lead performance.
Here's a screenwriting challenge for you: Your central character is a young child; his parents are important characters, but they have relatively little screen time. You need to be sure, despite their limited time in the movie, that they make a strong impact and come across as likable, devoted parents.
You could work hard on writing their few scenes to give them those characteristics, or you could take the lazy way out, as screenwriter Eric Roth and director Stephen Daldry have done: cast Tom Hanks and Sandra Bullock in the roles, and let the audience's affection for those actors do the work for you.
Similarly lazy, and bordering on vulgar and offensive, is the use of 9/11 as the movie's generic tragedy. The movie centers on a child's attempt to cope with the death of his father; for plot purposes, that death could have come about in just about any way imaginable. Admittedly, this one can't be blamed on Roth and Daldry; Jonathan Safran Foer's novel takes the hit for this choice.
Oskar, the child at the center of the film, is played by Thomas Horn, who is making his acting debut after having been spotted by the producers on Jeopardy! He is painfully bad, particularly in the more emotional moments. The movie relies heavily on his voice-over narration (another bit of screenwriting laziness), and he delivers every line with the same ponderous solemnity.
There are two marvelous supporting performances that almost make the movie worth seeing. Jeffrey Wright plays one of the many people who cross paths with Oskar during the film; one of the movie's worst moments, a particularly vile bit of cheap tear-jerking, is the monologue Oskar addresses to Wright. The marvel is that Wright, simply by sitting there and listening with absolute attention, sincerity, and conviction, comes damn close to making the scene bearable. It is a spectacular reminder that acting is much more than what you're doing when you're delivering lines.
Even better is Max von Sydow, who doesn't get to deliver any lines at all; his character has suffered trauma of his own, and as a result, he does not speak. But when he arrives about halfway through the movie, he is the first character who feels like an actual person, and the strength of his performance elevates Horn almost to believability.
But even with such solid support, Horn isn't capable of the performance this movie needs, and since he is the indisputable star of the movie, it collapses around him. Even with a better actor in the lead, though, I don't think the movie works, and I'd still be offended by its use of a national tragedy in so cavalier a way.