Particularly considering the uproar over the Department of Justice's demands for just this kind of information from Google only months ago, AOL's actions demonstrate a shocking disregard for user privacy. Search terms can expose the most intimate details of a person's life and, in doing so, cause great harm.
Consider just a few hypothetical situations. Would you want your employer or credit company knowing that you searched for "how to file for bankruptcy"? Would you want anyone to know you searched for "HIV positive clinic," "breast cancer health services," or another illness-related query? What about "rape victim" or "depression" plus "counseling"? What about searches that reference your political or religious affiliation, or your sexual orientation?
Though the data was associated with random ID numbers, that information could still be connected back to an individual given enough clues. Consider, for instance, what vanity searches for one's own name or MySpace profile could reveal.