Two big changes IMO:

1. Size- In past generations, it was normal to have heavyweights who were 205, 210 pounds. Holyfield was really the last of the smaller great heavyweights. Now that you have guys who are 250 instead of 205, of course they're not going to be quite as fast or throw as many combos as the smaller guys of the past.

2. Talent- Everyone always wonders where all the American heavyweight talent went. It's simple...the Ali, Foreman, Frazier, and Norton of this generation are all around, they're just playing in the NFL. If this were 30-40 years ago, guys like Ray Lewis and Michael Vick would have become heavyweight fighters instead of football players.