1. He is not motivated by the acquisition of power. This is what disqualifies most presidential candidates from my serious consideration. To crave power is to be unworthy of it. Gary Johnson obviously has different motivations, else he would never in a million years sought the nomination of the Libertarian Party. This is a party which historically (read: since 1972) considers 1% to be a very encouraging result in a presidential election.
2. His goal is not to divide the electorate. Both Barack Obama and Mitt Romney must absolutely depend on fragmenting the American people into an un-reconcilable "Us vs. Them" frenzy if they are to succeed. You have but to watch a few minutes of a speech by either man (which, coincidentally, is all that the non kool-aid drinkers can stomach) to see this plainly. Gary Johnson, to be successful, must reach out to reasonable people of every political persuasion. He holds views that transcend the left-right divide that both major parties require their candidates to swear allegiance to. Can you even imagine a Democrat or Republican presidential candidate who can make a reasoned argument for both gay marriage and gun rights? Good luck with that. While their may be few who agree with him on everything, almost everybody can agree with him on some things. He can become a figure of national unity, as opposed to the figures of national polarity we usually end up with.
3. He will not be a slave to corporate interests, unions, or existing political power structures. Because he is a Libertarian candidate, he is not going to draw huge campaign contributions from traditional sources. Thus he will be beholden to no one as President. Furthermore, as a candidate who is running on principle as opposed to expediency and demagoguery, he is unlikely to draw such support even if he somehow gains nationwide popular support.
4. He is a man to be taken seriously. Though it is an unfair characterization, Libertarians have a reputation as being, well, kooky. Gary Johnson is by all accounts and appearances a sane, sober, reasonable human being; neither an extremist nor a milquetoast. As a two-term governor of New Mexico he has the right combination of real-world experience without the Washington insider stink. He has run a successful business and is not a career politician. And for crying out loud, the guy runs triathlons and has climbed Mount Everest. That's pretty cool, no matter what your political beliefs.
5. It would be a shame for all the energy and ideas of the Ron Paul campaign to go to waste. Whether you like him or not, you must admit Rep. Paul been a breath of fresh air in the stale world of American Politics. A man who says what he believes, not what the pollsters tell him is popular. A man who transformed the way political fundraising is done. A man of principle running in a contest historically dominated by weasels in human form. Murky accusations of racism have been the only tarnish on an otherwise spotless reputation. Gary Johnson can capture and multiply that energy. Neither mainstream candidate has that ability.
Bonus Reason: Even if every above reason goes against your grain, you have to concede this: Gary Johnson will make the interminable presidential debate season much more palatable, if not completely un-awful.