I've always been skeptical of the emphasis placed on first year grades. And essentially I still am, but I've now heard the first argument that convinced me they might actually be a proxy for future success in the profession. Here's what happened:

I was having lunch with a judge and we were talking about how short a window new associates get in which to prove themselves at law firms. And the judge said, "That's why grades, especially 1L grades matter so much in hiring." This is the point in the conversation where I made the Scooby-Do "huh?" face. 

She explained: law firm partners are quick to judge their new associates, not because they don't think all their new hires can eventually become competent successful attorneys, but because they're interested in the ones that adapt to new surroundings and assignments the fastest. Same goes for judges. Clerks generally only serve one-year terms. Of course there's a learning curve, but you hope for a clerk who starts out relatively able to produce their work without major revision and/or tutorial. 

1L grades serve as a proxy, not for hard work or writing skills, but for an ability to accurately assess a particular set of demands and meet them. Most law students' grades improve over time. This reflects multiple factors, including a learning curve for the process of law school exam preparation and execution. Employers want those with the steepest learning curve.

I think the judge's argument was that legal employers are interested in those students who are thoughtful about process and procedure in general. Quick to figure out what kinds of information will be relevant to an assigning partner and what will be superfluous. Quick to realize how an opposing counsel works and how best to work with them. Quick to see how an office operates and how to function within it. A person who approaches their work with an eye for both substance and procedure.

None of this has anything to do with the comprehensiveness of one's research, the style of one's writing, or the long hours spent doing either.  Those things matter too. But whether you're prepping for your first year of law school or the bar exam or your next assignment, it's good to take a step back and really ask yourself not just whether you know the answers to the questions you'll be asked, but also how to deliver those answers effectively.