A Drum Major should be confident and respected. He or she should also be respectful. They should be hard on the band/corps without being unreasonable. Ideally, they should be well-liked before becoming a dm. If you're in a band like mine, where we only have one drum major regardless of how big we get, they should be a rising senior so that someone else will get a chance the next year.
Also, there's the obvious stuff, like being able to conduct, keep time, etc. Another helpful thing is the ability to project your voice without wearing it out. If your band goes to contests where the judge the drum major, you should have a sense of showmanship. But not something over the top.
Another thing to remember is that whoever chooses the drum major (be it the band director, another instructor, or the band itself), you've been trying out since the minute you've met them. People who become Drum Major display leadership qualities from Day 1. They know what they're doing and where they're supposed to be, and if they don't, they ask before it's too late.