Obviously the only legal qualifications are those listed in the Constitution. That would bar me from office (I'm not yet 35), but most of those in this forum would be legally qualified. However, given the importance of the role, most of us expect much more.
If we needed brain surgery to survive, we would presumably want to choose a doctor with some experience with surgery -- preferably more than a few years of experience, and preferably with a great success rate and sterling academic credentials. Heck, even if we were having our house worked on, we'd want a contractor with a lot of relevant experience. Why should we expect anything less with a role as crucial as president of the world's most powerful nation?
Here are the qualifications I'd like to see, with parenthetical comments:
1) High intelligence (This is tough to gauge in this setting, but academic successes, and verbal and debating skills can give you a sense of a candidate's intelligence).
2) Tempermental fitness (You don't want a hot-head with his finger on the button, but you also don't want someone so unsure of himself that he's paralyzed when action is called for. Ideally, you have a very curious person, who is open-minded and even-tempered, a person who doesn't come apart under stress, humble enough to defer to experts on technical questions, but secure enough to be a natural leader. You also want a detail-oriented workaholic).
3) Ample experience in high elected office (Ideally, that would mean Senator, Governor, or VP... but a House member or Mayor of a sufficiently large city would be good, with sufficient extra years. Using the rule of thumb that it takes seven years to master a skill, when you're fanatically diligent about it, someone with seven or more years of experience in high elected office would be preferable).
4) Ample experience in an executive role (Ideally that would mean VP or Governor, but it could also be Sec. of State, Presidential Chief of Staff, four-star military commander, mayor, or at least some lesser role within an administration, or an executive position in business. Once again, seven or more years would be ideal).
5) Some preparation for the role of Commander in Chief. (Ideally that would mean lots of experience as a senior military officer, but could also include working as a governor with authority over the National Guard, work in the DoD or defense industry, work as a civilian military analyst, or committee positions overseeing the military, in the House or Senate).
6) Great skills as a communicator (Success as president requires convincing the people that your ideas are good ones, and the ability to communicate effectively is key for that).
7) Knowledge about the rest of the world (An ideal President would have spent a lot of years abroad, studying or working or serving as an ambassador, and would have extensive experience traveling. He or she would be multilingual and would have a talent for communicating an American perspective, in a positive manner, to a foreign audience).
Now, my thoughts on how the three candidates stack up on those grounds:
1) High intelligence: This is the toughest to say, but my guess would be that Obama is the smartest, followed by Clinton, followed by McCain. McCain's miserable academic experience at the Naval Academy isn't terribly inspiring, whereas Obama and Clinton both have outstanding academic records. Between Clinton and Obama, Clinton's the clearly superior debater, but something about Obama's superior academic record and his manner of speaking suggests to me he may be smarter than her.
2) Tempermental fitness: I'd say Clinton comes in first here, based on how ridiculously even-tempered she's been when facing a media lynching throughout the campaign. She also strikes me as open-minded, detail-oriented, and hard working. Obama would come next. He seems to have a very natural leadership style and no obvious tempermental warning signs. McCain would come in a distant last place, based on his terrible temper.
3) Ample experience in high elected office: McCain is far and away the most impressive here. Clinton would be next. Obama kind of embarrasses himself on this one.
4) Ample experience in an executive role: I'd put McCain and Clinton in a tie here, based on his executive role as a military officer, and her important role within the Clinton administration. I'm unaware of any executive-style experience for Obama.
5) Commander in Chief preparation: McCain is far and away the most qualified, based both on his military career and his many years taking a leadership role in the Senate on military issues. Clinton would be a distant second, based on having also served on the Senate Armed Services Committee.
6) Great skills as a communicator: Obama would come in first on this one, Clinton second, and McCain close behind her.
7) Knowledge about the rest of the world: I'm not sure here. Clinton has tons of experience traveling the world as an unofficial ambassador as First Lady, so I'd probably put her first. Obama would be a close second, based on his own personal background living abroad. McCain's ages in the Senate probably have involved enough traveling that he is pretty familiar witht he rest of the world, but I'd still expect him to place third here.