With destiny being an mmo fps, they will have that competitive aspect, and leader boards, but I can see them setting up something similar to the way blizzard has set up pvp in the past on wow with arenas and having the rewards cash out in the same way, because destiny is going to be a gear based game, so not everyone is going to be using the exact same gear leveled and spec'd the exact same way...so creating ranked playlists like on halo just won't work the same. In halo it was based solely on skill with weapons that everyone could have access to. Destiny is gonna be based on skill and strength of gear, gear yhat everyone may not be able to attain.
They have systems so that overpowered weapons aren't too overpowered. One of these systems is that weapons start out with very low ammo and from what it sounds like we might not even start out with special or heavy weapon ammo so that snipers can't just get to a vantage point and then kill every one. They have to fight for those areas and it sounds the ammo well stay in place in drop in the same places for each game but will change game to game.