Avoiding (for the moment) discussions of Freemasons greedily working to subvert anything resembling Christendom, the first thing a Christian needs to be aware of is that Freemasonry's principles are opposed to those of Orthodox Christianity, hence membership with Freemasonry is impossible for someone struggling to be a genuine Christian. It would be similar to the conflict that would be involved with claiming to be an Orthodox Christian, yet be integrally involved in a movement that affirms the equality and churchliness of contradictory (even blasphemous) sects of so called Christianity.
Freemasonry itself has the characteristics of a religion - however, it is one which does not make exclusive claims on it's adherants, so they are free to be a part of whatever religion they please (something like old pagan Roman polity - "worship the gods of the state, but otherwise you're free to adore whatever gods you please.") While the convinced Freemason will hardly see a problem in this, Orthodoxy obviously does, and ultimatly that is all that is important.
As for other problems of Freemasonry - some would include it's occult entanglements (the severity of this some dispute, but few would dispute that it is at least present). It's general secrecy, and the taking of oaths which are immoral, is another problem. While I know many would say "oh, you're taking it all too seriously", the point is no Christian should be swearing oaths of these kind (indeed, we're discouraged from swearing oaths at all, let alone towards immoral ends) - that people treat the oaths others take with such frivolity is more a sign of our own decadence, than a demonstration that swearing such oaths is problematic.
I'm sure most men who become Masons do such simply to establish professional relationships, get connected, etc. A few, I think, are deeply attracted to the principles of Freemasonry (at least the ones which are well known and give for public consumption.) What I would remind people of however, is the well known (at least, historical) malevolence of Freemasonry. True, the more public manifestations of this could be found in Roman Catholic parts of Europe, but the accounts are still troubling. If you can come across a copy (though much of it can be found online), Albert Pike's Morals & Dogmas (Pike is something of a hero of Freemasonry to this day, and this work is considered a Masonic classic) I think the wickedness of this organization becomes further apparent.
Seraphim