" to talk down (to an audience), to lower one's discourse to the assumed level of their intelligence;"
So when someone talks down to someone else, the speaker is assuming their audience is not as intelligent as they are, and they simplify their language or vocabulary choice accordingly. Adults often talk down to infants, for example, and it's that sort of context which makes the phrase a negative one. To talk down to someone is considered insulting and demeaning. The indications are that Mr. Jackson intended it to be a very uncomplimentary assessment of Senator Obama's style. I can't think of a situation in which "talk down to" would be used with a positive connotation.