On paper, a show I should dislike immensely: period setting, focus on character, generally dark tone, regular violence. Keri Russell and Matthew Rhys (and Noah Emmerich and half a dozen others) are superb, a fine example of the way television still credits itself for artistic breakthroughs when reaching a level considered basic competency in a film--the actors are all good and credible in their parts.
Speculative reasons for enjoying:
1. The setting provides a neutral backdrop. 1980s suburban DC is just a place, chosen for the types of stories it makes possible. The show never deliberately stages either of the kinds of nostalgia that frequently appear in such shows: no "man, people were crazy back then!" and no "things were so much better!" of the sort that Mad Men can never quite manage to shake.
2. Female characters comma Large Proportion of. Mad Men may be Peggy Olsen's show, but after Joan or Megan there's a long way to go to another fleshed-out female character, and the show will periodically make clear that Peggy's professional success comes from her willingness to Work Like A Man. In the Americans, each male character has a female equivalent, and some of the female characters have no male equivalent. The female characters get all the same shadings of motivation and interest that the male characters get, and are much more rarely treated as obviously lesser because they are women. I suspect a body count would be evenly divided between men and women.
3. There's a moral weight to the violence, some large portion of which happens offscreen. Whenever someone dies, people are upset. If one of the characters did the killing--even if they have done a lot of it--the show depicts them struggling with themselves for having done so. All the spies know themselves to be morally compromised, and spend a reasonable amount of time struggling with that: they want to win, but not at any cost.
4. It's not a spy show in the same way that Archer is not a spy show. Archer is a radio program for English majors who enjoy literary and vocabulary jokes that uses a spy show as its pretext. The Americans is a show about marriage and relationships that occasionally does very intricate chase scenes.
5. Marriage comma A Credible Depiction of. The general options are to stick two happy people together and mostly ignore them, or generate a bunch of soap opera relationship complications. Friday Night Lights had its moments of tension between Coach and Mrs Coach, but they were ensemble players in a much larger story. The Americans excels at putting Elizabeth and Philip at cross purposes and then forcing them to work together, so that openness and vulnerability lead way to actual changes in their relationship, that they literally and metaphorically fight over how their family is going to be, how they will raise their kids, what their KGB-mandated relationship is supposed to consist of. They are parents, and they are married, and they have time apart from each other for things other than work.