CU was when they changed all the stats on weapons and armor, limited your armor and weapons choices, greatly increased item deacay, among a million other changes.
NGE was when they chanaged it all to fixed classes (Including Jedi as a starting class) , removed decay from the game altogether, only one experience type, and you got most of your XP from quests. Made attack manual aim and attack, rather than a locked target that you could attack even while afk.
They basically updated it to be like the "next generation" of MMO's were at the time. Which, people who wanted that could play one of the new MMO's. People played SWG because it was not like the others.