After having read two and a half of his books and watched hours of his stuff on youtube, it evades me what fascinated me in the first place. He communicates philosophy with wit, charm and insult (always the best combination), but on the bottom line - to me - they bring no insight. I'd say Zizek's popularity has more to do with the poverty of leftist thinking nowadays (in that way he's not so different from organic food, GMO-activism, anti-nuclear-demos, and the like)...
The grand weakness of all of Zizek's work is ultimately his idealism: He sees ideas/ideologies/the structure of thought/psychoanalytic theory as concrete things in human beings that are universal. So, he is essentially making loads of human nature arguments - and never presents a shred of evidence to back them up. He just starts with "I claim..", something like "that ideology functions objectively," and then he makes a joke to illustrate what he means and goes on. Also, when he does analyse such an "ideology", he doesn't justify why this should be one coherent thing in the first place.
In the end, maybe I would have liked Zizek to write similarly to how he does, but more concretely. Because ultimately, if you ask him what the left should do today, his answer is: "nothing, think". Wow, how revolutionary :)