These effects are on theyre own pretty, and also not a big problem, but when 7-8-9-10 different ones stack up then its pretty annoying and far from pretty.
A marker on the ground where a boss mechanic will hit is not of much use when its covered with 10 layers of player effects.
Also different trinket effects, proc effects etc are way overdone when it gets many of them like in a big raid scenario.
Those effects should be toned down drastically or a interface option to tone down or turn of other peoples special spell effects.
As its now its just way to annoying.