When is government intervention in the economy justified?

Please note, before we start, that my question here isn’t whether the government should interfere. The view that the gov’t should never, under any circumstances, coerce its citizens is, I believe, a seperate debate. My question is when the gov’t should act. Taking as given that the gov’t must sometimes step in, I’m interested in the criteria the gov’t should consider before it acts to interfere with market forces.

To be honest, I’m interested in this because I’ve met, and read the writings of, several economically-conservative people whose ideas of when the gov’t should interefere coincides remarkably and unsurprisingly well with their own economic niche.

Lawyers are my best example.

There are large barriers to entering the law market. The purported purpose of this is to insure that all people who are allowed to give legal advice and representation are of the highest quality, but not to be ignored is the large economic incentive of current lawyers to reduce the number of future competitors.

Let me pause here, because I don’t mean to sidetrack discussion with this. I support certification for lawyers, for the purpose of having high quality lawyers. Because of the importance of good legal representation, I believe that gov’t inteference in the law market is justified. But interesting to me are people who support interference when it helps them, but who seem vehemently opposed to other forms of gov’tal interference.

This isn’t necessarily an inconsistency on anyone’s part, but the apparent conflict of interest is, in my opinion, too obvious to be ignored, thus this thread. For those who favor little interference in the economy, what does it take for the market to go too far? Under what circumstances should the gov’t regulate market activity?