On the buff side, it preserves rage. It also triggers a couple talents to generate resources in combat: rage and runic power.
On the debuff side it does a wide range of terrible things to us.
It stops all natural health regeneration. The only exception is troll regeneration at only 10%.
It prevents entering stealth without vanish.
One of the worst is that it gives mobs the ability to track you anywhere for the duration of combat. You simply cannot hide, they always find you.
You cannot eat or drink while in combat.
You also cannot switch between specs, mount up, or use flight paths.
The application of this debuff is also strange.
It may come from attacking a mob and will last until it dies. Or it can come from being attacked, lasing until it dies.
It may also endure even if you are far away and the mob has little interest in you, just as long as it is fighting someone.
It can be applied across an entire raid zone, even to players who are nowhere near the enemy.
Hostile actions involving pets or totems also trigger combat, even if the pet or totem are nowhere near the player; which then allow the NPC to perfectly track the newly debuffed player.
Against NPCs the debuff is non-durational, lasting as long as the NPC 'wants', since given time and distance it will lose interest. The debuff can even be triggered and maintained under odd circumstances, such as destealthing near a sapped mob.
Against players it is instead a short debuff, around 10 seconds, triggered by hostile actions to or from another player. The mere presence of other players will not trigger combat, not even under the circumstances of leaving stealth near a sapped enemy, or even blinding an enemy and simply waiting out the debuff.