One problem I've found is a tendency to assume that if you don't agree on means, you must not want the ends.

Example: "If you don't support single-payer, you must not want people to have access to health care." Or from the other side, "If you don't support the war in Iraq, you must not want America to be defended from terrorism."

There are many variations on this.

A liberal may believe that good health care requires the state to run it. A conservative may believe that the private sector will deliver good care if allowed to. But both want good health care for Americans.

A conservative may believe that the road to defeat terrorists in the Middle East must eventually run through Iraq. A liberal may believe that Iraq is not the only linchpin, and might point out the role of Saudi Arabia, Syria, Iran and others in the war. But both want America to be safe and strong.

It's hard to keep the truism in mind that being wrong doesn't make you a bastard, and being right doesn't make you more virtuous than the other guy. God knows I probably fall off that wagon as often as anyone. But it's so FRUSTRATING sometimes... ;)