This sounds like a great idea if implemented well, but I just see the potential for a gigantic amount of noise. I mean the set of all topics is mostly highly-specific and uninteresting.

If you would subdivide everything into clusters and nominate a small set to represent that cluster, and only then suggest things that are highly dissimilar (but not opposites), this might be extremely useful. If one already feels strongly about a topic (health, politics, religion, etc) I'm sure that person would know where to seek to get opposing points of view.

If I am interested in programming or math material online, getting recommendations about conflicting problems or programming methodologies would not be very useful. However, getting info about topics like flying gliders, rock climbing or philosophy would be great.. (I'm picking things I'm familiar with, but I would have loved to find out about similar interests and hobbies through suggestion)