You have picked a big question! I guess one place to start is an examination of the relationship between Crusoe and Man Friday - this is one of the key areas of debate for this kind of question. The relationship between Crusoe and Man Friday has been examined eagerly by a number of critics, especially in recent times postcolonialists, who have variously seen their relationship as depicting the worst of slavery or a genuine mutual respect.
Their relationship certainly seems to be ambiguous and open to interpretation. There are times when it appears to be almost based on a father-son type of intimacy, but others suggest that there is a clear master-slave element to their relationship. This latter perspective is reinforced throughout the text. For example: "I made him know that his name was to be Friday... I likewise taught him to say Master". The naming of slaves by their masters was key in Defoe's times, and the fact that Man Friday never knows the true name of his master indicates an attitude of extreme superiority.
Man Friday, however, appears to be incredibly grateful to his servitude to Robinson Crusoe, and places Robinson Crusoe's foot on his head in a manner that "seems was in token of swearing to be my slave forever." Thus Man Friday's "slavery" might have been in gratitude for being saved by Robinson Crusoe. Crusoe certainly seems pleased to have Man Friday with him: "I took him up, and made much of him, and encourag'd him all I could..." yet we are left unsure whether this is due to any essential goodness in his nature or just sheer relief at having someone else to talk to, for "they were the first sound of a Man's voice, mine own excepted, that I had heard, for 25 years."
Thus there are two main views: the master - servant relationship, as evidenced by the authoritarian way in which Crusoe treats Friday, and the father - son relationship, in that Crusoe does seem to genuinely care for Friday's well-being.