I heard once that Orthodoxy, unlike Islam, seems to be designed to avoid gene pool stagnation. The fact that we aren't permitted to marry our cousins is an example. Another example is the previously mentioned relation through godparentage, Meaning man can't marry a women if the women's father is the man's godfather, even if they are not related by blood.
This might not seem like a lot now, but imagine being a man from a small village community. After a few generations your options for marriage would be limited, and it would cause you to go further afield to find a wife, thus spreading the faith. I'm not sure that spreading the faith is the intention of this prohibition, but it is a result. I think it really helped Christianity avoid tribalism that Islam still suffers from.