I love me a good debate, the problem is, on the internet they more often morph quickly into circlejerks.
What used to be a topic where people argued and expressed their opinions over certain topics becomes a thread for people who are on one side, and one side only. It eventually gets to the point where 2-3 posts are made against a single other, each given a like (or whatever the equivalent is) by the others making the same argument. It gets to the point that some people feel uncomfortable posting in the thread, because one side has taken over and other opinions aren't welcome.
It's a problem even here, and if you don't believe me, just check any D&D thread over a day old.
Not to mention people crying "logical fallacy" over every post they don't agree with just to sound smart and try to end an argument then and there.
Another problem is that hypocrisy tends to run rampant, and if you point it out, people pretend like it doesn't matter or that both arguments are still logical and relevant. Similarly, everyone tries to pass opinions or very swayed statistics as facts.