The ones that bother are me are things like the transparency on the window borders.
It's transparent, but it blurs the background.
It serves no purpose - if it was transparent and you could see though, I could guess there might be some practical application.
OS X has a lot of eyecandy, some of which is totally useless, but a lot of it does have practical value (e.g. expose looks good and I use it all the time).