1) It is not soapy. "Downtown Abbey," I love you, but when your plot lines begin to sound like something out of a bad romance novel or a telenovela you've gone too far.
2) It is not extensively sexist or racist. Yes, I understand the importance of showing viewers a less idealized more realistic portrayal of the past, but after awhile I grow tired of listening to the sexist and racist jibes in "Mad Men."
3) It is well acted. It has Ben Whishaw in it. Need I say more? Also Romola Garai, Dominic West, and just about everyone else are excellent.
4) The costumes are beautiful and suit each character. Bel's jewel toned pencil dresses and skirts, her silk blouses and headscarves. Her trench coat. Freddy's tweed suit in Season one. Lix's Katherine Hepburnesque trousers. Marnie's full skirted dresses. These are just what immediately comes to mind.
5) The sets, which were done by the same woman who did the sets for "The King Speech," are chock full of good period appropriate detail.
6) The jazzy understated soundtrack.
7) It doesn't center almost exclusively around one person (especially not season two). Cough Mad Men cough. That gets boring. There is only so much about we can learn Don Draper. His psyche and troubled past are not six seasons worth of (good) material.
8) It is aesthetically pleasing to watch.
9) It is intelligently written.
10) You can't end a series (spoiler alert!) with one of the more major characters critically injured. It's cruel.
11) It inspires in me the sort of fanatical obsession I have not had for anything since the first season of "Sherlock." Translation: it's good.
P.S. Perhaps it seems like I am bashing "Mad Men" and "Downton Abbey." I enjoy them, I truly do. I am just pointing out the traps "The Hour" has avoided. Also I have seen other period television series. They just are not mentioned because this is getting long enough as is.
Photo source: http://icapturetheperiodpieces.tumblr.com/post/35695315305/the-hour-tv-series