OhSnap is an addon that will help you to improve your PvP awareness in arenas and in battlegrounds.

It has 4 priority settings, going from high (1) to low (4):

1. Spell alerts, Huge font: Polymorphs, Cyclone, Roots..
2. Annoying buffs, Large font: Divine Shield, Ice Block, Resting..
3. Noticeable buffs, Normal font: Evocation, Lichborne..
4. Profitable debuffs, Small font: Hypothermia, Weakened Soul..

In arena the addon will show information from all hostile targets you can find. Out of arenas it will show information only from your target.

With the GUI you can configure the messages, change font size, different font style, remove icons, remove comments, add spellnames.. You can also enable test mode on go watch ie. duelers whacking each other outside of Orgrimmar.