Since some people in this thread are asking and nobody has explained:

The surplus xp or "Fatigue" system was in place at launch and quickly removed within the first few major patches. Basically as you gained more xp (called sp back then) on a single class, your gains started to be penalized by a percentage. You weren't penalized on other classes though. This was to encourage players to try many classes and to discourage players from leveling a class too quickly. This wouldn't have been so bad if there was more content and the xp system was as we have it now, but at launch this was widely considered unacceptable, so they took it out.

BONUS: a video explaining it http://www.youtube.com/watch?v=abE09-tqhoM