The common man of the 16th century had several possible reasons for a conversion to Protestantism. The first possible motivation was a spiritual one. The Catholic Church had steadily continued to vest spiritual power in the clergy, perpetuating the "spiritual monopoly" the ordained clergy had in comparison to the laity. The clergy were the only ones who could communicate with God and absolve sins on behalf of their parishioners. Protestantism, on the other hand, allowed every individual the opportunity to communicate with God through prayer. This ability may have excited the 16th century European who had very little volition in other aspects of his life.

Another more practical motivation for practicing Protestantism was its strong relation to capitalism. Protestantism viewed prosperity on earth as a way to revere God (read Max Weber's "Protestantism and the Spirit of Capitalism"). Conversely, Catholicism generally regarded the wealthy as inherently sinful and in many cases the wealthy were patrons of the arts in order to absolve any possible sins they perpetrated while amassing their fortunes. The ability to amass a fortune without compromising the ability to go to to heaven would have appealed to the common man of the era.

Finally, the matter may not have been one of choice for the common 16th century man. In Davide Cantoni's "Adopting a New Religion" he surveyed 119 territories and found that 3 factors influenced the adoption of Protestantism: nature of a territories rule (ecclesiastical or not), its size and influence (less powerful=more likely to be Protestant), and its proximity to Wittenberg (Luther's hometown). The second of those three brings up an interesting possibility. Powerful and influential areas were powerful because of connections to the Catholic Church and the Papacy and were less likely to convert because they enjoyed the prosperity the Church brought them. Weaker and less influential areas were tired of the corruption (indulgences, power-brokering) of the Church and chose to turn to Luther's Protestantism as a way to return to the scripture.

Sources

Davide Cantoni. "Adopting a New Religion: the Case of Protestantism in 16th Century Germany"

Aaron Ketchell. March 2014 Lecture. "Protestantism"

One of Luther's biggest ideas in his theology is that there should be no intermediary between man and god. A person could pray directly to god without the Church or religious figure, one could beg for forgiveness, absolve sins without going to the Church, let alone paying for it through indulgences. Another critical point is that Luther helped translate the Bible (although it had been translated beforehand, Luther's version had the support of the Protestant movement so it became more famous). Luther was against the Pope having the sole right of scriptural interpretation or/and appointing councils that could create doctrine. What you have is ONE man with the power to tell everyone in the Catholic World what God meant, or appoint a council that would tell the Catholic World what was doctrine. Oh, and all the sources were in Latin so the layman couldn't understand it. All men were created equally under god (rather, all men are equal before god [another crucial point for Luther]), therefore all men can interpret the bible for themselves. In order to do that, the bible should be translated into their language and they should be taught how to read.

So with Protestantism you have:

1. Easier theology/ religious practices
2. Sermons are held in your own language
3. Encouraged/ taught how to read
4. Encouraged to have your own interpretation of the bible
5. Don't have to pay for salvation

With these incentives, the average European would find the Protestant movement appealing, though some times it depended on the ruler.

If you're interested in the subject, I'd encourage you to read Luther's An den christlichen Adel deutscher Nation (To the Christian Nobility of the German Nation) where he presents most of these points.