Jude wishes upon his readers specific blessings. His salutation is not the same as the apostle Paul and some of the other writers used. He specifically chooses "mercy, peace, and love," as all three are vital in times of apostasy.
He asks for mercy because they probably needed to repent. His whole reason for writing the epistle stems from the fact that they had begun to get lax, allowing false teachers and false teachings in. They needed God's mercy as they began to repent.
He wishes them peace because, obviously, a major result of apostasy is war and division. Remember, his brother writes in James 3:18 that the fruits of righteousness are produced in peace, and these people were not producing the fruits of righteousness for two reasons: false teachings and war. Thus, they needed peace
Finally, he includes "love," the prime virtue. They needed love because it would take love to resolve this situation—and not just love for God but love for one another. This is the agape form of love, not just phileo— not just caring for one another but setting the mind to do God's will for each other and for God.
Richard T. Ritenbaugh