First, there are misconceptions about excommunication which stem from the complexity of the subject matter itself, and the lower level of catechesis that today's Catholics bring to the discussion. It is harder for people to understand the notion of excommunication if they have insufficient appreciation of the underlying concept of sin, or of what membership in the Church implies, or what kind of authority the Church has from Christ, and so on.
And the most common misperceptions are: First, there is the idea that excommunication kicks one out of the Church. That is not right. There are ways to cancel one's Church membership, but excommunication isn't one of them. Imagine a felon serving a long prison term; he's in prison, but he remains a citizen bound by the laws of his country. Say, he owns property upon which he incurs taxes while in prison, he still owns the property and is still liable for the tax from prison; if he commits a crime in prison, he can be prosecuted for it, and so on. A felon loses certain important rights, obviously, like freedom of movement and the right to vote, but he is still a citizen. Similarly, an excommunicated person is still a member of the Church, but he or she has lost certain key rights attached to Church membership and is cut off from many of the activities and benefits of the Church.
The second misconception is that people who die in a state of excommunication go to hell. Maybe they do, and maybe they don't, but we don't know with certainty either way. In any case, the Church does not claim to exercise jurisdiction over the dead, and one's final fate is determined by God based on the life one leads. Of course, appearing before God for judgment in the state of excommunication from His Church on earth is not a good thing.
The third misconception is this: many people think that, because a given Catholic committed an action for which automatic excommunication is the penalty (for example, heresy, schism, abortion), the penalty was actually incurred in that case. Canons 18, 1323, and 1324, contain a startling list of factors that mitigate or even remove liability for canonical crimes. Now taken individually, these exceptions to penal liability make sense, but when read as a whole, as we have to do, they make it much more difficult to determine whether an automatic excommunication was actually incurred in a specific case.
Excommunication is not about politics or ideology; at root, it's about sin. The mainstream media doesn't understand what sin is, so they surely aren't going to understand what's behind excommunication. Now, there are lots of examples of sinful behavior out there, but only some of them, in general the worst ones, are also crimes under canon law. Those are the kind of things to those wanting the immediate excommunication of pro-abortion politicians, I have to say that canon law simply does not read that way. To make a long story short, an excommunication for abortion has to be linked to a specific abortion and, given the structure of American government and medical institutions, one simply can't link a given legislator's vote with a specific abortion within the limits of causality recognized by canon law. Now, I like to think that the words of law generally mean what they say. If we distort the words of penal canon law to the degree necessary to make legislators fall within the present terms of the abortion canon, we would do violence to the text of the law, and that's always bad; distortions in law tend to come back and haunt us in other contexts that are the subject of excommunication.
Do Catholics have an obligation to know their rights and duties under canon law? Yes. Knowing the law, knowing one's rights and duties, is not sufficient for leading an honorable life, of course; but knowing the law makes leading the upright life easier. One knows what's expected. For too long, canon law, even though it is an incredibly important source for knowing one's rights and duties as a Catholic, was neglected. Today we are paying the price for that widespread disregard of Church law right and left. But things are slowly changing. The value of stability, the wisdom of experience, the basic commitment to dignity, all of these are waiting for us in canon law. The more people learn about it, the more they see how valuable it is, and the more they want to learn.