I know some elaborate effects can slow you down a bit in a 25man raid. But honestly, some of the spells in this game look just awful. Most players have something stronger then a duel-core with built in graphics and can handle a little more pretty in game than a blue or red circle on the ground.
I know they mentioned for WOD that they wanted spell effects for an individual to look nice while others see a basic effect, but why not allow us to tune it in settings for us to choose what we want to see. If some people have trouble in raids dealing with heavy spell/particle effects let them turn it down. But for us with a little more umph in our computers, let us crank it. Cause i'll be honest, while the dungeons look pretty, the spells and effects seem downright boring.