The critical moment is one of self realization. It happens at the Fall fashion show for Christian Dior. The internal reflection had been building for some time in terms of wondering how her working for Miranda had changed her, how her social world had dwindled to only Miranda, and how her work in the fashion industry was actually doing more harm to herself and other women. When her family pleads with her to come home, she ignores them, but after given another assignment by Miranda with the usual callousness, Andrea has enough and tells Miranda what to do. This moment, punctuated by a great use of language, helps to form the climax as it is one where Andrea knows she can never return. Miranda fires her on the spot right there and while Andrea is hampered professionally, she has redeemed herself personally. This is the moment of actualization, the threshold of revelation and makes for the emotional climax of the novel.
For me, the climax is the part in the novel where Andrea, the young assistant to Miranda Priestley at Runway magazine, decides to leave and live her own life. She does so in a burst of anger and publically, so she fears that her career as a writer is ruined, but finds that her outrage has made the papers and that many are willing to buy her freelance articles. Andrea also meets an editor with whom she becomes friendly (hinting that there may be the opportunity for future full-time work at her magazine) at a teen magazine. The editor tells Andrea that she also worked for Runway before editing her own magazine, so there is an instant bond of knowing what the two women have been through having slaved for Miranda.
Although there are other parts of the novel which are tense, they all help to build up to the final burst of high tension--Andrea's public outburst and leaving/being fired from her job.