All armor should be able to be unlocked by everyone. The acheivment system should be only for campiagn. Armor skins though, could unlocked through multi-player acheivments.
Yeah. I see what you are saying. Maybe have more military look and feel to them. Like Reach's. Combine both Reach and 4s arnor, so people like you have what they want and people like me have want they want.