There are several specific reasons Senator McCain appeals to Indpendents like myself.
1. He works across the isle, and he gets things done. Whether it is the gang of 14, campaign finance reform, or anti-torture legislation he is a very successful legislator.
2. He has an impecable human rights record. Whether it is spear heading anti-torture legislation, promising to close GitMo, standing up for the monks in Burma, or speaking out against the genocide in Darfur, if there is human rights offense Senator McCain is front and center speaking out against it.
3. He's a straight shooter. Going into the Iowa primaries saying he doesn't support ethanol subsidies, or going to Michigan and telling people that the jobs that have gone overseas are not coming back is politically very gutsy. Senators typically have difficulty running for president because with a long record an opponent can often spin their record to highlight contradictions. However, Senator McCain's record is remarkably consistent, and he says what he thinks even if it isn't what the pundits think he should do.
4. Experience/Intelligence: Go to his events; he answers any and all questions in a very thorough way. He gives details, he allows for follow ups, he lays out his plans, and he simply knows his stuff.
5. Independence/Conviction: People like Senator McCain and Senator Lieberman have taken a lot of heat for not following the party line. They stand up for what they believe in and even if you disagree with them on issues you have to admire their backbone.
6. Walks the Walk: While Senator McCain's military experience alone may not be what makes him qualified to be President, it certainly helps that he has a strong understanding of the military. Also, you can't help but respect his service and his family's service past and present. The fact that his sons currently serve in a time of war illustates that he walks the walk. His sons didn't take a pass because they had a famous and influential father, they illustrate a McCain family committment to service.