Tom Hanks does a great job of portraying a kid in an adult's body (he was nominated for an Oscar for this role). His childlike innocense while getting just about every kid's wish (growing up) and having to deal with issues of adulthood, such as getting a job, finding a place to live, and handling relationships really shows. Predictably, these situations produce comedic results, though the predictability isn't that big a problem.
It's interesting to notice how Josh changes after having sex. Aside from his jumping for joy in the office in the morning, he suddenly becomes more businessman-like and gets angry at his friend (who's a kid) while he talks on the phone to some business associate. This is a pretty interesting comment on what adulthood does to someone. In short, they stop having fun and appreciating what being a kid is like. That's the message here--no one should be in a hurry to grow up.
An intersting ethical issue the movie brings up is the story Josh makes up to explain his disappearance--that he's kidnapped. Obvioulsy this causes a whole lot of stress and anxiety on his mother's part, who believes a crazy person has her son. Was this right for Josh to do? He does try to explain what happened, but then he resorts to this kidnapping scheme.
I've heard the Jennifer Garner movie 13 Going on 30 is pretty much a remake of Big but with a girl instead of a boy, but I haven't seen that so I can't compare the two. Overall, Big was a funny, light-hearted movie that's worth watching. It stands up to later movies, since the message is still relevant today (as evidenced by Garner's psuedo-remake).