I agree that normalizing gear makes the arena setting far more competitive. When you even the playing field as much as possible, that is when the skilled players can truly shine. Leaderboards, vanity gear, mounts and pets, titles, rewards (could even be in the form of gil) for placing high enough for particular "seasons" of PvP - all of these can motivate PvP. Gear is not necessary in arena-style PvP.
