Abandoning the pretense of objectivity does not mean abandoning the journalist's most important obligation, which is factual accuracy. In fact, the practice of opinion journalism brings additional ethical obligations. These can be summarized in two words: intellectual honesty. Are you writing or saying what you really think? Have you tested it against the available counterarguments? Will you stand by an expressed principle in different situations, when it leads to an unpleasing conclusion? Are you open to new evidence or argument that might change your mind? Do you retain at least a tiny, healthy sliver of a doubt about the argument you choose to make?
Much of today's opinion journalism, especially on TV, is not a great advertisement for the notion that American journalism could be improved by more opinion and less effort at objectivity. But that's because the conditions under which much opinion journalism is practiced today make honesty harder and doubt practically impossible. Like the mopey vicar in Evelyn Waugh's novel Decline and Fall, who loses a cushy parish when struck by a case of "Doubts," TV pundits need to radiate certainty for the sake of their careers. As Lou Dobbs has demonstrated, this doesn't mean you can't change your mind, as long as you are as certain in your opinion today as you were of the opposite opinion a couple of days ago.
But if opinion journalism became the norm, rather than a somewhat discredited exception to the norm, it might not be so often reduced to a parody of itself. Unless, of course, I am completely wrong.