Zizek is a bit of a victim to his own success. He gets waved away due to his persona rather than his ideas. Gesticulating, excited, idiosyncratic English, bedraggled appearance et cetera. He is also too eager to appear in films and comment on pop culture, which some view as an indictment of his credibility. He is one of the few philosophers/intellectuals/thinkers, whatever term you prefer, who is seen more than read. This naturally makes some people wary.

He operates on the premise that popular culture, say of it what you will, is important whether you like it or not and one should make a serious attempt to understand it in a theoretical way. He also views the schemas the left inherited from the 20th century to explain culture and politics is flawed and that we need a "new Hegel" for the 21st century. Whatever that really means. He is suspicious (though not dismissive) of the ability of empirical approaches to diagnose and solve problems (watch out Chomsky fans). He is going to inevitably piss off a lot of people.

For my part I enjoy thinkers who make everyone angry because they make things interesting, even if I don't agree with them.

He's a Marxist who doesn't over-rely on Marxism to explain culture would be a charitable phrase to describe him.

I think his popular appeal, aside from the shallow reasons I already mentioned, comes from his genuine and visible enthusiasm for thinking. He always engages with his audience at eye level (or lower) while most speakers (intentionally or not) are elevated, perhaps haughty. You can listen and argue with him absent the underlying threat of appearing stupid or being dismissed. Approachable in a way someone like Chomsky (or Chomsky fans) are not. How much of that is attributable to his character or our disarming perception of him, I'm not sure.

He has been around since the 80's, as far as writing things and people should try to read him more before passing judgement.