I have to agree with Olivant. It was the haircut. Vincent wore his hair slicked back much like Michael in the first two films. Michael's hairstyle was reminiscent of Vito. That spikey hair in GFIII made Michael look like walked right out of a bad 1980s MTV video.

I would also add that Michael was too outspoken in the third film. Like his father, Michael always kept his true feelings quiet in the presence of others. The "old" Michael would have never made that joke about listening to Tony Bennett records, screamed at Vincent to "shut up," made confession, and he would not have had that conversation with Kay ("gimme the order!").