One of my all-time favorite classes was the debate class I taught in Chicago. My students were mainly from Korea, but also included at times a Brazilian business man, a Taiwanese father, and a Japanese sociology student. We talked about globalization, plastic surgery, cloning, genetically modified food, legalization of soft drugs and poverty, among other things. For them, the class was a laid-back atmosphere where they could practice their English by discussing current topics that they generally found interesting.
For me, aside from the fact that I was doing my job, I had a chance to talk about interesting and relevent topics with people from another country, much like I try to do when traveling. It was also interesting to me because while my students were all from other countries, they’d been staying in the US and so could take that experience into account too.
Make sure the aims are clear: if the goal is to have an interactive discussion, you may decide to make note of bigger or repeated mistakes, but don’t interrupt the discussion to have a talk on grammar.
Be aware that you might encounter politically incorrect attitudes. I personally find it very hard to know what to do in this case; my role is to provide a context for students to practice English, not to teach them what to think. It is also tricky in an inter-cultural context: are my views right just because I’m the teacher? A native speaker? An American? But at the same time, it’s hard to hear a prejudiced/racist/sexist comment and ignore it.
Consider assigning students a side to argue rather than letting them present their own views. With more serious topics, this can prevent it from becoming too emotionally charged because it doesn’t become so personal. Defending a view you don’t necessarily agree with also gives students practice in critical thinking.