Malcolm is elder son of King Duncan and newly appointed as Prince of Cumberland, known to be the holding place for the next King of Scotland. When we first meet Malcolm, he seems rather weak – he's standing around praising a brave and bloodied Captain for saving his life and rescuing him from capture. In other words, Malcolm's the kind of guy who seems to need rescuing. This doesn't exactly sound kingly, does it?
Malcolm's reaction to news of his father's death doesn't recommend him to be king yet, either; it only shows he's still feeling around for the best course of action. He seems to lack the experience to make him confident or capable. Only when he meets Macduff, who complements him in courage and experience, do we begin to see the seeds of power in Malcolm. In order to test Macduff's honor, Malcolm makes himself out to be a lecherous tyrant who's more interested in selfish gain than he is in the good of the kingdom. Everything makes sense again when Malcolm admits he's a virgin (not a letch) and was just teasing Macduff to make sure he was true to the cause of Scotland. (Note: This could be a nod to King James I of England, who was supposed to be "chaste" before he married.)
Malcolm's words at the end, praising and gifting his allies and damning his enemies, make it seem like he'll follow right in the footsteps of his dad: gracious and, for the most part, harmless. Even if Malcolm isn't going to be a tough warrior anytime soon, he has folks like Macduff to help out, so long as Malcolm can continue to make the speeches and be pure of heart, which we are sure he is.