Based on my own experience playing diablo 3 these past few years, and people's "I'm bored now that I've completed my _XX_ character" I really think that when you set yourself a goal in the game like "I will complete the hammerdin Crusader build, that looks really cool"... after the long grind to complete the build, many players find that they don't have any motivation to keep playing.
They accomplished their goal, but what keeps them playing beyond that? I remember first starting out I made this mistake way back making the Blessed Shield build. Sure I completed it, but I didn't feel like playing my crusader anymore after finishing my goal. If instead I had said "Hey, I want to beat my record of 2-player GR 40 with my buddy", and getting the Blessed Shield build was just a milestone... then I still have something to do after I complete the build.
Does that make sense? I think setting longer term goals then just "finish this build" is really important. Maybe I didn't communicate the difference between a goal and the means to a goal enough.
Thanks for the feedback nitrosnwbdr. I plan to update the guide for Patch 2.4 so I'll definitely try to make that clearer.