Hero worship means admiring someone for unholy reasons and seeing all he does as admirable (whether it’s sin or not).(My emphasis.)
Holy emulation, on the other hand, sees evidences of God’s grace, and admires them for Christ’s sake, and wants to learn from them and grow in them.
Read the whole thing for Piper's thoughts on how to navigate the tricky waters of pastoral popularity, signature seekers, and cynicism.