Bullet I appreciate the input. The idea of having achievements to unlock a particular armor is meant to encourage a player to use a weapon and become good with it. Lets say you are bad at sniping like I used to be, I mean I got owned by it and when using it. By repeatedly picking it up an using it and along with advice from friends and other players, I am considered a threat with it now. The sniper armor would only unlock after "x" amount of sniper kills, something repeated use would unlock. The upgraded version of armor would be for those would attain "x" amount of sprees of "Headshot Honchos" and so on with each achievable armor. This encourages use of different weapons initially and rewards the player who is very good with it. This can be applied to all armors unlocked by a weapon achievements.
Yes, the "flaming sword", "glowing or flaming body" would be temporary, losing the perk when you died and like you said would draw attention to the player on a streak that needed to be dealt with, which in and of itself, would make streaks a little harder to attain. The flaming sword would not have any more power or damage capability, it would be purely cosmetic as would the lingering flames left on the dead body. The same would be true with the "streak" flames the player would have had attained with the necessary spree or frenzy.
The idea, in a nutshell, is to encourage the use of different weapons. Remember also the social ranking that I mention. These armors and upgrades would be unlockable in the less competitive environment of social play as well where the competition is usually easier. A new thought here: The sniper variant could have a different look when attained in social as compared to ranked play as would the upgrades. This might encourage the casual player, once he or she acquired the social version and hopefully improved their skill, move into the more competitive ranked arena.
It may be hard for some to remember and for new players to accept who want it all made easy, but when I played match making in Halo 2 I mainly played Skirmish, which is now Objective. This seemed to be an easier game type for me to learn how to kill because I would stay near the objective and players came to me giving me more chances to kill. Later on I moved into slayer, then onto "ranked" slayer, doubles and FFA. I wanted to be good at the game, if only to kick my nephews butts who repeatedly owned me. I took the time to learn the game and ranks and armors meant nothing in the beginning.
Consider this, achievement unlocks give a longer life and a continuous challenge. There are still those who I know that go back to H3 to try and unlock a particular armor.