Attack ads and going negative

Nate Oman at CoOp talks about and links to two of the masterpiece political ads: Lyndon Johnson's 1964 "Daisy" and Ronald Reagan's 1984 "The Bear" (the first political ad that ever really stuck with me and still the best of my lifetime). Nate labels both as "attack ads," but I am wondering why (especially as to the Reagan ad). What makes something an attack ad--is it oversimplifying an opponent's position Is it mentioning (or alluding to) an opponent and his position at all? Is it a complete absence of any reference to the candidate's own positions in favor of criticism of the opponent's position? Is it that it is somewhat personal? Something else? And note that at least "The Bear" probably is not an attack ad under many of those definitions.

This is a narrow question subsumed in a broader one: What constitutes "going negative" in a campaign? This issue bothered me during the 2004 presidential election, when John Kerry was accused of going negative for ads and comments that criticized President Bush's performance in office. But that seemed strange (and unfair), because, by definition, a challenger runs against the incumbent's past performance and only can do that by talking about what the incumbent had actually done in his time in office. Is criticism of the opponent per se negative?

By the way, if you watch the ads, the length was cut in half from 1964 until 1984--and at 1:00, Daisy now feels really long.