There are many reasons why WoW is a great game, but to me, one of the things that sets it apart from other MMOs is the sound design. The music is top quality, the voices are entertaining and memorable, and the sound effects are relevant and informative. As a matter of fact, sound effects are critical to the way I play.
Even if I turn down the game music or environment sounds, I leave my sound effects turned way up. Why? Simple. Without them, I would be a less effective player. I rely on the casting sound to tell me when my heals will land and when I can cast the next one, and I listen for the thud of a tank's block or the clink of a parry to let me know when damage has been mitigated. My druid's inarticulate grunts tell me to back away from adds or to move out of the fire, and even when I'm watching the health bars instead of the characters, the game sounds tell me what spells my party members are casting.
I don't always recognize all these things literally, but the presence of Blizzard's sound effects gives me an idea of how the fight is going by sound alone. In the midst of a tense encounter, I can't always keep track of all the visual cues, but sound effects help me follow what's going on.
Do you use sound effects to keep track of what's going on in combat? Which sounds are the most identifiable to you? Without the sound effects, would you be as effective a player?>