Just to expand on what Katie said, Trials of Atlantis was an expansion that turned a lot of people off. It brought the game the best items and gear as well as special abilities called Master Levels to the game. People feel that it helped contribute the overall escalation of damage and shorter fights as well as seperating the haves from the have-nots...aka the gankers versus the casual gamers.
The classic servers did away with any of this. They still have all the other stuff in the game, including expansions that came after ToA, but there are no artifacts and Master levels (as well as some buff changes to eliminate buff bots). They are definitely better for casual players that want to have a chance to compete.