Here's what Wikipedia had to say about this: "[George Clooney] was reportedly asked to return briefly in season 8, to make an appearance in Anthony Edwards' last episode during Dr. Mark Greene's funeral, but Clooney declined because he did not want his cameo appearance to overshadow the departure of a beloved character on the show."

I understand what he was trying to do, but this really bothered me. Your best friend's funeral!! You show up. Come on now Doug!

Did it bother you that Dr. Ross didn't show up for Dr. Green's funeral?