The most obvious question these changes raise is "Why are stats being changed, and why now?" As the game has matured, we've run into increasingly complex issues with the current stat system. Many stats are inherently confusing, and the way they interrelate can feel convoluted. Attack Power, for example, currently translates to damage, but so does Armor Penetration. Defense provides five different statistical benefits of varying utility. Mana regeneration involves understanding multiple stats and rules and often ends up being irrelevant anyway. In addition, the difference between a "good stat" for a class and a "bad stat" can be extreme. Some casters want Haste but not Crit; hunters want Armor Penetration but not Haste. There are other overarching issues, as well, such as Intellect not being very exciting for casters despite it being a core stat -- and these are just a few examples.
Our ultimate goal is make gear a more interesting (and less confusing) choice by making each stat valuable to more players. While the reasoning behind some of the following changes may be clear, we understand that you may have questions about some of the less obvious alterations, and we'll do our best to answer any questions you may have here on the forums.