Tunes. They can make or break a game. Example: Prince of Persia-Warrior Within. They used one of my favorite bands to add just an epic heavy metal tone to the Dahaka fights. GodSmack for Life!
Guild Wars has a predictable habit of using Jeremy Soule for their soundtracks. That is because that man has fantasy in his soul. If you get a chance to, I am going to suggest that everyone goes out in a thunderstorm in game. Have a seat, dance like crazy, or just play with dyes in the middle of one. Because if this *below* is the default soundtrack for storms, I haven't heard a better storm song since Legend of Zelda:Ocarina of Time.