Defense is also important. I know some try to argue that armor beats all, but I know our healers tell a different story. Defense reduces or nearly eliminates the chance to be critically hit by a mob. The amount you need varies by level and by spec. At level 70, to be technically crit immune, you need the spec that decreases crit by 3%, and have a total of 415 defense. Without that spec, you need 490 defense. Of course, at lower levels these numbers are different and may not even matter much due to the level of incoming damage.
Note: I say technically immune, because there is a very very small chance that a crit will land no matter how much defense you have. Its around 0.001% if I recall correctly. Also, there are certain bosses who have been 'crit buffed' so that they have a higher amount of crit than normal. As I understand it, having a higher than maximum defense won't help any more in these fights, you just have to realize that you will get hit with a crit from them. Fortunately there are very few mobs like this.