The first parallel lies within the play itself. I deeply appreciated the subtle resemblances between the Hotspur-Glendower and the Hal-Falstaff scenes. In each case, a young man spends a significant amount of time taunting a self-important, older man who is such a verbose blowhard that the audience almost instinctively sides with the young man. Hotspur, whom we are inclined to respect due to his high spirits and his feats as a warrior, is so easily provoked and wears his self-esteem so blatantly on his sleeve that his needling of Glendower, although perhaps justifiable, appears pointless, hasty, and ill-advised. (It may, in fact, prove to be his undoing, as Glendower fails to come to Hotspur's assistance when it is most needed - a dereliction perhaps exacerbated by the younger man's abrasive heckling.) As a result, although we like Hotspur at the end of the scene as much as we did at the beginning, our respect for him has diminished considerably.
In contrast, consider the Hal-Falstaff exchanges. Hal, already characterized as a wastrel, deflates Falstaff's pomposity with a controlled barrage of pointed wit, causing us to begin to admire him for his self-discipline (at least in conversation) and to sense that there may be more to him than meets the eye. Moreover, Falstaff, unlike the humorless Glendower, is a worthy adversary, brimming with wit and self-awareness, and the fact that Hal can not only hold his own but also maintain his composure suggests a self-awareness and a deliberately cultivated detachment from his debased surroundings, which prepares us for his eventual transformation just as much as his soliloquy about the sun.
The other parallel, between plays, is more closely related but certainly less significant. Lady Percy, in her efforts to obtain information about the impending rebellion, delivers a speech that closely resembles Portia's speech to Brutus in similar circumstances. Their subsequent conduct, however, is different. Portia, the stoic Roman, cuts herself in the thigh to prove her ability to keep a secret, while Lady Percy, a hardy warrior's bride, attempts to break her husband's little finger and force him to talk. (As I mentioned earlier, this isn't of great importance, but it is interesting to note how a great dramatist can utilize similar materials to achieve very different effects.)
Overall, I am once again astounded by Shakespeare's remarkable command of voices in this play. Hotspur, Falstaff, Glendower, Hal, and Mistress Quickly all employ language in highly distinctive ways, and even the casual banter of the servants in the stable yard is vivid and characteristic. I am also impressed by the expert and seamless integration of poetry with prose, history with comedy, and rhetoric with wit.
By the time he penned Henry IV, Shakespeare not only had mastered all these elements but also knew precisely how and when to blend them. This is undeniably the work of a master.