Here's some interesting information about how the game handles EXP. Level 90 is the half-way point EXP-wise, in case anyone wondered.

Hmmm.. I'm not so sure that information is completely viable. If I remember correctly, the experience system took a bit of an overhaul when the 1.10 patch was released. Judging by the last update of this guide, I don't think it is entirely accurate.