In conveying the message of Islam to a non-Muslim, it is usually not sufficient to highlight only the positive nature of Islam. Most non-Muslims are not convinced about the truth of Islam because there are a few questions about Islam at the back of their minds that remain unanswered.
They may agree with your contentions about the positive nature of Islam. But, in the same breath, they will say - "Ah! But you are the same Muslims who marry more than one woman. You are the same people who subjugate women by keeping them behind the veil. You are fundamentalists, etc."
I personally prefer asking the non-Muslim upfront, what he feels is wrong in Islam. I prefer asking directly, what they feel is wrong in Islam with their limited knowledge, whether right or wrong and from whichever source it may be. I encourage them to be very frank and open and convince them that I can take criticism about Islam.