A) Original actors replaced. The original American cast of professional actors was replaced from AvP99 to AvP Gold Edition (2000). Gold Edition instead re-recorded all the monitor FMVs with the Rebellion crew. The reason, I suspect, was most likely monetary: since they were re-releasing it, the original actors would have to get paid again. They wouldn't have to pay the Rebellion crew extra (or at least, not much) for their time recording, since they were the developers, too. This is the most likely reason I can justify. AvP Classic 2000 is an AvP Gold port, and therefore it retains that game's FMVs. You can replace them with the originals via ModDB.
B) Blood effects reduced. Any change in blood decals between AvP Gold and AvP Classic 2000 has to do with the port job itself and is unintentional. You can change the blood decals in the settings, and you can also play through on a SuperGore mode that essentially doubles the gore to insane amounts in all versions.
C) Violence in cutscenes. The only cutscene that was ever censored was an original gory FMV showing a marine getting his head clean blown off by a shoulder cannon shot. While awesome, it is also incredibly graphic (bordering on hilarity) and also the first thing you see before starting the predator campaign. I'm sure Rebellion and Fox decided the cutaway was best and let viewers imagine for themselves what it must've looked like. As with the FMVs with the original AvP99 cast, you can also download the uncensored version of the cutscene via ModDB.
If you have a German version of the game, there may be censorship because of the country's strict video game policies.