Ninety percent of all stories are based on the Idiot Plot -- you've seen it: any conflict that could be resolved in three seconds if not for the fact that everyone involved is an idiot. (Seinfeld based an entire series on this idea, but at least in their case it was intentional.)
Then there's the other nine-point-nine percent, in which the plot involves a real problem, worthy of the main characters' attention (and, therefore, the audience's as well). NYPD Blue, ER, and other shows with life-and-death professions fall in this category. (At least, the good ones do.)
Picket Fences, that last tenth of a percent, is the rarest of the rare, in that each episode seems to start off with the Idiot Plot, everyone getting stirred up by minor little things that should be solved easily...but then something very unusual happens. You slowly realize that this issue is much deeper and more complex than it first seemed. Pretty soon, the most innocuous of personality quirks has expanded into weighty moral, ethical and philosophical territory, and everyone's got an opinion worth considering, whether you agree with them or not. Ain't no slam-dunk solutions in Rome, WI.
I don't know how David Kelley did it, but he handed us the single most compelling drama series I've ever seen. There are shows I've enjoyed more, with characters I liked better, with action that was more satisfying (action in the story sense, not just violence), but few that compelled me to think like this one did. Even when things ended badly, horrible travesties of justice unfurling as we watched, I couldn't help but think, "I hate it, but I can see their point."
(On the downside, the quality of the series also took the biggest, fastest nosedive in TV history in its last season, when Kelley left the show and let someone else revamp it...but that's sometimes the penalty for such a high level of excellence in the first place, I suppose.)
A few other notes: No review would be complete without mentioning Ray Walston's Emmy-winning role as 'The Judge Of All The Earth', as I think of him, and I would be remiss if I didn't add that this series had one other quality achieved only by the finest of dramas: it's funny as hell.