The left is dominated by pseudo-intellectuals who typically know enough on a given subject to be obnoxious but not enough to discuss the issue with any depth.

The right has a significant number of social conservatives who are content to let you think what you want but quietly think you're going to burn for it.

I find both pretty obnoxious but it is the left that tends to be more overtly arrogant in its pronouncements.

The typical left-wing tactic boils down to this:

IF you disagree with them then it is only because you're ignorant. If only you were as enlightened as they are you would certainly reach the same conclusion on the topic they have. It often never occurs to a left of center debater that there may be other conclusions someone can legitimately reach. 

My advice: If you find yourself calling someone ignorant for no other reason than that they don't share your opinion, it's time to re-evaluate your position.