The way they approached the problems with Sunwell in 3.0 and 3.1 seemed really good. All of the content was easy to finish, but fights like OS3D, firefighter, yogg+1, and algalon were engaging for a long time. After that the hard modes stopped being significantly different from the normal versions of the encounters, which makes them a great deal less exciting. The lack of excitement in high-end raid content should only impact high-end raiders, though, so this probably has very little to do with the people who are leaving the game.