Web searcher satisfaction.
Yes, you read that right...satisfaction.
So how on earth does Google measure satisfaction? A recent SEOMoz post has the details.
The basic idea is measuring the conduct of the user once they click the search link. For example, a searcher may type in "Springfield injury attorney." A list will pop up, and the searcher will click one of them (most likely one of the first three that pops up).
What the user does after she opens the link guides Google's assessment of that user's "satisfaction" with the site. If the user only waits 2 seconds before hitting the Back button and clicking a new link, then the site's ability to satisfy readers may take a hit.
So what does this mean for blogging or law firm SEO? Try to get readers to stay longer. This can be done in many ways, from ensure proper technical site details (i.e. quick loading speed) to have designs which are user-friendly and instantly intuitive.
When it comes to law firm blogs or web content, the same rules apply. Break the material up, use visuals, and in other ways make the material pleasingly to eyes so that the reader may actually stay and absorb the content.