Highly polished, elegant vernacular prose; fresh, descriptive, conversational vernacular
Although Boccaccio follows the well-established tradition of modestly berating his own literary abilities, there's no real way that he truly believed that he was a writer of small talent. Just look at the rhetorical structure of the opening lines of the Prologue, which uses a formal metrical trope to create a pleasing rhythm: "To take pity on people in distress is a human quality which every man and woman should possess [...]."
The writing style is highly varied, from the flowery and formal medieval speech of the brigata in the introductions to the stories and their aristocratic characters, to the earthy and streetwise language of The Decameron's bakers, weavers, maidservants, and stoneworkers. Shmoop's favorite example of this is the different ways that the brigata vs. the story characters talk about sex. In the storyteller's own commentaries in the stories, it's all "tasting love's ultimate joys," and "that sweet pleasure for which they yearned above all else." For the everyday people in the tales, it's "lifting the wolf's tail," "putting the Devil in Hell," "tilling the field," getting the mortar ready for the pestle, "shaking her fur," etc. etc. Boccaccio's the maestro of the euphemism.
So you'll recognize the conscious elegance of passages on friendship, suffering, sex and mortality—as well as the down-to-earth and practical language. The humor's brilliant—it's fresh and contemporary. The stories themselves are filled with realistic description of the daily lives of the stories' characters that adds authenticity despite some very improbable plots.
Boccaccio had some doubts about writing in the vernacular. He was never totally convinced that writing in his native everyday Italian was as respectable as writing in Latin, and even his idol Plutarch criticized his choice, saying that he only skimmed the book. There was an opinion of many literary people of the time that if someone couldn't read Latin, they weren't worth the trouble to write for. In Boccaccio's later years, after a religious awakening, he wrote that he regretted writing The Decameron. But as his most popular and widely-copied work—a medieval bestseller—we like to think that he didn't entirely abandon his masterpiece.