As others have said, Moss has always taken plays off. Guys don't just start taking plays off all of a sudden, it is something they've done since gradeschool and is a habit they are never going to break.
Moss is as talented as any receiver in the game, but his tragic flaw will always be his attitude and effort, and that's why he will never be as great as guys like Jim Rice, Tim Brown, Art Monk, Cris Carter, Michael Irvin, etc.