The enmity meter shows how much enmity you have generated against the current mobs you're fighting. Green for safe, red for full hate, and so on. This works great for dps and healing, but for tanking, it's rather counter-intuitive. Every mob should be attacking you, and having a bunch of red bubbles indicating so only helps marginally.
I'm interested in having the meter work entirely backwards as a tank. Green when you have hate, yellow and orange when someone is pulling hate off of you, red for if you've lost it. Having to tab through targets to see their enmity levels means that you're essentially in the dark to hate being generated on every other mob in the fight. This way would allow you to react almost instantly when someone pulls a dangerous amount of hate, and seems to make a lot more sense