There's no question about it: Hatchet is first and foremost an adventure story. The book has all the elements—and then some—that make the adventure genre so exciting and fun. Dangerous situations! Mortal peril! Plane crash! Hurricane! Bears and wolves! Moose attack! (Moose attack?) In fact, if you're not on the edge of your seat at least a few times while you're reading Hatchet, you can pretty much assume that you're clinically dead.
If Hatchet were just an adventure story, though, it probably wouldn't have struck a chord with the number of readers that it has. What allows the book to really shine, and to catch the reader's attention in the way that it does, is that we really care about Brian from the start. Why? Because he's already experiencing some conflict even before all the awesome, adventurey (yes, thank you, we know that's not a word) action starts happening. Even though Brian's the only character to really appear in the book, there's still a family drama taking place in Brian's life and inside his head.
Finally, Hatchet is a children's book. Although it definitely appeals to both adults and kids, it's aimed at and speaks most directly to young people. Besides the fact that its main character is a thirteen-year-old boy, Hatchet is concerned with lots of things that are often deeply interesting to kids, and that adults (sadly) can often lose touch with: the natural world, animals, creativity, and problem-solving. You know, things that kids just have a knack for.