World Of Warcraft: Why are blue items better than Epic items?

This question was asked in the Wow General forum and Nethaera from Blizzard chimes in to give the answer:

We have already started working on balancing the equipment in game a bit more with patch 2.0.10 with the corrections to socketing. There were some items that had to be adjusted to more appropriate values which meant some were adjusted down and others were adjusted up. This is going to be something we plan to continue working on over time in upcoming patches as we evaluate items that may not be living up to what we wanted them to be or are more powerful than originally intended.