While knowledge and technical skills are very important, they do not go very far if the customer has an unpleasant experience in dealing with the HelpDesk. It goes without saying that if the customer's experience is unpleasant, the support tech is not having a great time either. But it is very important to be able to size up where the customer is at in order to effectively deal with him or her. You must, as much as possible, put yourself in his or her place. Probably the majority have only vague technical knowledge.
Many customers are too afraid to show their ignorance.
So you may think you have done a wonderful job when, in reality, your explanation went way over the customer's head. You should be able to detect a language problem by the way the person speaks. One thing that is very important is that you speak slowly and enunicate clearly. Everybody has difficulty distinguishing a b and a p, for example, over the telephone. Again, don't assume that the customer is going to clearly understand your jargon.