At first glance, Byron's Don Juan contains all the fancy-sounding words of the early nineteenth century. Words like "bereft," "behold," and "assail" are ones that you would find in any great epic poem of English literature. But that's exactly Byron's point. He takes an absurd story like Don Juan's and dresses it up in some of the most exquisite poetry ever seen in English. Thus, instead of using his powers to elevate English culture, Byron actually uses them to cheapen this same culture. He's writing verses that no one else could ever hope to duplicate, and then he's crumpling them up and throwing them in our faces.
Check out this example from Canto V: "Her first thought was to cut off Juan's head;/ Her second, to cut only his—acquaintance" (5.139). Here, Byron is taking the epic form of poetry and using it to describe a queen who wants to cut off the main character's penis. Using a nice-sounding word like "acquaintance" just mocks all the English readers who would try to protect themselves from gross subjects by dressing them up in fancy words.
In addition to this elevated vocab, Byron also shows his chops with advanced sound techniques. His favorite of these is alliteration. Check out stanza 7 in the dedication: "Your bays may hide the baldness of your brows,/ Perhaps some virtuous blushes; let them go." Here, Byron busts out the B sounds to bust on Bob Southey, his poetic rival. Once again, our poet takes the high road (sound-wise) to dive gleefully into the gutter.