Actually able to carry out the responsibilities of the office. Not to my exact wishes on every policy matter, not deserving of it, not the best possible person - just plain able to carry out its actual day to day duties. If Al Sharpton were president tomorrow he'd need new pants. If Kerry were President tomorrow existing staffers leafing through existing rolodexes would smoothly transfer several thousand educated liberal academics and politicians and New England socialite cronies into each executive department, each with a policy background and position papers on every issue.
I'm not saying I'd like his policies, I wouldn't. But he would not be out of his depth, cluelessly lost level, incompetent. It would be his competence at implimenting policies I disagree with that would be the problem. Not his - not knowing what the Federal Reserve is, who to appoint to the State Department, what is to expect an intelligence briefing to consist of, who he must hear out before deciding which matters, and the like.
There is such a thing as basic qualification for the office. Kerry has it, not everyone does. Bush had it - he had been governor of Texas, run multi-million dollar businesses, seen the White House under his father and how it functioned, etc. Cheney had it - he ran the Pentagon in a major war, ran a major corporation, ran the White House as chief of staff for Ford, etc. Dennis doesn't have it, Sharpton doesn't have it, LaRouche doesn't have it, and a lot of other people who pretend they want to be President, when they really just want to be on TV.
It is a useful distinction to maintain, not something to trash for the sake of talking points in one campaign season. Kerry's problem is that he is a liberal who fully reflects the liberalism of a major political party, that is wrong on the substance of important national issues. He was wrong about the things his party was wrong about in the past, too. This amounts to saying he is a liberal democrat and liberal policies aren't good ones, particularly right now.
It does not amount to saying he is one of the clownish characters above, who are *unqualified* (ordinary English meaning of the term, which should be obvious to practically everyone regardless of party) to be President.