Well, she's a 9/11 Truther, so her credibility immediately sinks through the basement for me.
She also thinks the CIA assassinated or assisted in the assassinations of MLK Jr. (plausible) and Tupac (rather doubtful). I think the incident with the Capitol Police reflects poorly on her sense of professionalism, as well.
My overall impression of McKinney is a reasonably liberal Democrat who has wonky positions that undermine her otherwise workable platform.
For example, her on Al Gore:
"Why don't you support native peoples in Colombia opposing oil drilling on and near their traditional lands?
Also, you're a racist, because your campaign doesn't involve enough black people!"
- Al Gore turns and looks confusedly at his black campaign manager, who shrugs -
She also apparently hangs out with the New Black Panther Party, which is vaguely troubling in a "who would think that's a good idea?" kind of way.
But really, it all comes down to her being a 9/11 Truther. I certainly can't say she hasn't done anything good—she definitely has—but being a Truther is such an outrageous proposition for a high-level government official that it boggles my mind.