haha! *high5s* Night Lords are awsome.
but you are all speaking of three sides of the same coin. Iron Warriors, Word Bearers and Night Lords. i would certainly protest dubbing the Iron Warriors as a 'minstay' that title would probably go better with the Black Legion, or Word Bearers. the Iron Warriors only claim to mainstream is that McNeil seems fond of them
and given honsou's hated of the Ultramarines, many codex-centric players might assume they are a main chaos force.
to elaborate, allow me to compare and contrast those three.
These three Legions (Iron Warriors, Word Bearers and Night Lords) all serve Chaos Undivided. they all have individual members that fall to one god in particular, and these individuals are looked down upon. they are all evil. this gives them a some what better baseline for comparison than those who serve one god in particular (since they are heavily skewed in whatever direction their dark lord fancies).
Of those three, only the Word Bearers are truly servants of Chaos Undivided, they are pious, and work for the glory of Chaos, lead by uber chaplain-sorcerers, they, in my mind atleased, are the 'ideal' chaos marine legion. unless you are deeply familiar with the innerworkings of one specific legion, when someone says 'Traitor Legion', the image that comes to mind is far closer to the WB than the IW or NL.
The Iron Warriors always struck me as merely giving lip service to Undivided for the perks, and a excuse to be assholes, nowhere, in any story, piece of literature, scenario.... *anything*, have i ever found a example of the IW serving chaos for Chaos's sake, they are not pious..... they are greedy
, they serve themselves, they offer glory to chaos in exchange for personal favour, not out of any kind of religious obligation.
For a third point of contrast, the Night Lords are even *less* 'chaos'y than the IW are. they dont even serve chaos per-sey, they might be better described as Chaos-Aligned Rogue Marines, than Chaos Marines. there has been significant degeneration of their ideals over the 10k years since the Heresy, but fundamentally, they feel that the Imperium is a injust society that must be torn down so that a just human empire can be forged (make no mistake here, they are capital-E Evil and specialise in terror tactics, scaled up to the point that they once scared a entire planet to death, but they are very definitely Lawful Evil, their Primarch Cruz, had a alter ego as the Night Haunter... 40k batman, and nothing more need be explained
) and see Chaos as the only means to achieve that end... if you are familiar with the factions of the Inquisition, they akin to the Xanthites in their thinking, Chaos is a tool. they however plan to use it to tear down the corrupt Imperium. (the more chaos-corrupted sects of the Legion may cross more into IW-style thinking though, ive never even heard them really even praise chaos with much enthusiasm)