If you are asking this question, I will assume you are feral spec.  If you are serious about tanking, I will further assume you have a full three points in Survival of the Fittest, which is a talent in the Feral Combat tree.
The normal defense cap is 490 for any character.  However, druids that have the talents mentioned above have a reduction in melee damage, which reduces the effective defense cap to 415.
Since this is the case, defense and stamina should be your primary concerns until you hit 415 defense.  After that, other attributes will take importance over defense, for the most part (such as dodge, etc.)
And remember that one of the most important considerations for feral tanking druids is armor.  We get a huge buff to AC, so this is a primary consideration as well.