Well...I'd have to disagree with you here! I think it's quite possible to disagree with a person over many different topics and still not be nasty or shallow/insincere. Just because you disagree, it doesn't mean you despise them or their ideas. I think, perhaps unfortunately, that many people think exactly that though, which leads to some of the confrontations we see on this forum.
I think part of the problem is the nature of the beast, so to speak. Our faith is such a massive part of ourselves; such a private, personal, intimate expression of what we think matters most in this life. And when we are confronted by someone who challenges those beliefs, we tend to lash out. After all, to accept that those challenges could have a sound basis would be tantamount to accepting your foundation had cracks in it.
I think the key to this is to just be sure. As in, confident in the one who holds the truth. So what if your understanding is perhaps flawed? God's truth will stand, regardless of your human failing or intellect. If you are sure of what the bible teaches, and that that person over there is wrong, then that's sad for them. Sure, have a conversation about it. Learn about what they think, grow in your own understanding about what you believe. But ultimately its not up to you to hold the foundation of truth and the universe. It's up to God. And I think that should free us up to converse with others about what they believe without the need to get really nasty. Sure, try and point to God's truth, if you are sure...God calls us to be beacons of light, and to be passionate about it! But if we can't get through, or we can't answer something, or someone else is being nasty...it's really on them, and on the strength of God's truth. And I think he can take it. We can walk away secure, knowing that despite how the conversation went, who may have "come out on top", we presented our view and shone our light. That's all we can do, right?