The larger aspects of the PT are more often than not, simply not ever mentioned in the OT.

-Chosen One/Balance to the Force

-Qui-Gonn or Windu

-Warning of excess attachment to people

-The term "Sith" is thrown around much less and not even mentioned on screen in the OT.

All of these take up major parts of the Prequel Films, and while some not being brought up could be dismissed as a part of the passage of time, that none of them really highlights the differences between the two trilogies.