The title may suggest a generic romance, but Loving actually refers to Richard and Mildred Loving, the real-life interracial couple whose 1958 marriage was illegal in the state of Virginia (and nearly two dozen other states with anti-miscegenation laws), and who fought for years to have said law overturned.
It's always ironic to see foreign actors cast as historic Americans – Australian Joel Edgerton plays Richard; Irish-Ethiopian Ruth Negga plays Mildred – but hey, Daniel Day Lewis was Lincoln and David Oyelowo was MLK, so what can you do? And the casting here is actually spot-on: Edgerton and Negga are dead ringers for the Lovings, and with writer/director Nichols' script eschewing any big speeches or showy moments, they disappear into their humble characters completely.
Now, a word on that humility: Nichols, et al were clearly devoted to capturing the quiet dignity of the Lovings, even if Mildred was a woman of few words and Richard a man of almost no words. The intent is to show us that they were not rebels or superheroes but simple country folk whose love felt normal to them, and who just wanted to be left alone. (ACLU lawyers, played in the film by Jon Bass and a drily funny Nick Kroll, were the ones really promoting their case.) As a result, the Lovings in Loving come across as an everycouple, almost as generic as the film's title. Because we're rarely shown the two together, enjoying each other's company or bolstering each other's spirits in dark times, the film doesn't convince us, on a dramatic level, why preserving their marriage was so important, beyond the obvious reasons (they were harmless, they had kids, no couple should have the government telling them they can't get married). One issue is that Nichols could never dare include a "meet cute" scene between Richard and Mildred, because they first courted when he was 17 and she was... 11. That's an icky detail about a couple you're otherwise supposed to root for, so Nichols avoids any mention of their ages and we are left to assume that the Lovings were thirtysomething, based on the ages of Edgerton and Negga. (In fact Mildred was 18 when they married; Richard 24.)
Nichols, who's shown a talent for suspense in his earlier features Take Shelter and Midnight Special, breaks up the slice-of-life plotting with a handful of paranoid scenes in which Richard fears racist attacks upon his family. Without giving anything away, your reaction to such scenes depends on how much melodrama you expect from a movie like this, and how soon you accept that Nichols is keeping things true to life.
Loving is a sweet film, very nicely shot, with rich period detail and atmosphere. I even got a little choked up at the end. It was only after it was over that I realized I needed more of an emotional connection to the characters. Your mileage may vary.