You go to the eye doctor and he will have you behind the giant eye machine. "Better ... or worse?" Cha-chick, another lens pops in front of your eye. "Better ... or worse?" People in WoW often end up doing this with their gear. It's kinda sad.
Your players should NEVER have to resort to third party applications or data sources to make decisions about their in-game character.
People in WoW run addons and consult external (non Blizzard) web sites to decide what gear is better for them. No wonder so many people see raiding as hard core! Nothing in the game can actually tell you how expertise rating will help, or hurt you. Nothing tells you if 25 agility is better than 25 critical strike rating.
If someone has to go to the Elitest Jerks forums and read five or six posts about their class before they can understand what gear is best for them, you have failed as a game designer. You have basically divided your community of players into knowledge rich and poor.
I see two ways to repair this in a game. One way is to make the game easier so that such decisions become less important. People can show up for raiding in whatever they want and manage to get through it. The other way would be to actually show all the data to people in a format that helps them make good decisions. Basically, every piece of gear should come down to "is this more or less DPS/healing/tanking?"