While various reviews mention Castle Crashers' simple story, the storytelling is well done. For example, at the beginning of the Marsh level:
You walk into the marsh and see that skeletons have killed a peasant. As you begin to fight the skeletons, two other peasants peek out from behind the terrain and watch your fight. They look at one another and nod, then leap out from the terrain and begin to assist you.
The designers could have just dropped in some NPC assistants; instead, they chose to tell a story that gave meaning to the NPCs' behavior - the peasants help you because you avenged the death of their friend. The game is full of little visual and gestural details that help the player understand what's going on - the sort of storytelling details that Scott McCloud writes about.
The game is also a good teacher - here's how you are introduced to sandwiches:
You reach a door that you cannot break open. Enemies run onto the screen intermittently, but steadily. Each time you kill one, it yields a sandwich that goes into your usable-item inventory. In fact, the unbreakable door itself is shaped like a sandwich. Everything points to the inevitable conclusion: try out one of those sandwiches, and see if you can't get the door open.
There are many good game design lessons to be learned from Castle Crashers. I recommend it.