As far as expectations, for the proactive job I would negotiate goals with the publicist for number and stature of media outlets I wanted coverage from. e.g.(blog coverage, trades, or national TV and newspaper coverage). I would expect them to spend their time understanding the product and then pitching the story to reporters, and then to field press calls and handle logistics (press releases, arranging interviews, etc.)
Expectations for a reactive publicist are not as clear cut. I'd want them to control damage, but I can't think of a concrete metric for that.
In either case, I'd make sure they had news or reporting experience before hiring them. If they don't understand how the media works from the inside, they won't be as effective as someone who does.