By far Holden's favorite word is "phony" and "phoniness." He is an extremely critical and judgmental character and he sees the phoniness of people at times with alarming clarity and often times he is able to describe it in very humorous ways. Perhaps this is why readers resonate with him. However, he is also very blind, because he, too, is phony and this is something that he does not consider at all. This shows that he is immature.
Holden is also fond of using slang. For example, he says things like: "that drives me crazy." By this he means that he dislikes something with vehemence. This shows that he is an angry youth with a lot of pent up emotions.
Another example of Holden's use of slang is his use of the word, "kill." This can mean something good or bad. This shows something about his personality as well. It shows that Holden blows things out of proportion, just like many young people. As you can imagine, with age comes calm.