Usually when people say "speaking my mind" they mean venting. Venting is bad for civil discussion because it doesn't take others into account. It's only concerned with discharging one's own feeling. That hits others, who happen to have different associations than you do, with unexpected force, provoking discharges of their own—and so on and so on. Everybody feels justified because it always feels like the other hit first.

This is how we get religious flamewars and other degenerations, so it's important that we all learn to inhibit our ventings and transmute them into civil, substantive discussion.