Language and Date
I enjoyed this version, but there were a few things that bothered me. For one thing, it felt too modern. Part of the reason for that, I think, was the foul language used by some of the characters almost from the beginning. You could make a case that, by the later scenes, stress was leading to a breakdown of standards. But I don't believe a gentleman in 1939 would have said "s**t" at the dinner table, with or without ladies present, as Armstrong did before the murders began. It may be hard for us to believe today, but nice people simply didn't talk that way in social settings.
In general, Armstrong's behavior was ungentlemanly in a way that seemed inappropriate for a man of his class and era and that does not match his character in the book. (As much as I like Toby Stephens, I thought his character was a weak point. He was a nervous wreck from the beginning and verging on hysterical several times. It was overdone, and, again, not like the book, where Armstrong is outwardly quite conventional, although tormented and alcoholic in private.)
Similarly, in the book, Lombard is a gentleman, or least from the "right sort" of background. And yet, very quickly the TV character's language becomes ... not right for the period. Also, an outwardly decent man of 1939 would probably not have wandered around at length in a dressing gown that he scarcely troubled to close over a bare chest, much less in a towel slung so low on his hips it threatened to reveal at any moment whether he was shaved, trimmed or natural. Not that I'm complaining. ;-) I'm not saying that such a thing could not happen; human nature was as variable then as it is now. I mean only that it is behavior with a modern flavor, not offset by any remark from a character to put it in context.
While we're at it, a pet peeve of mine: Men in 1939 did not trim their chest hair, at least not in such an obvious way. (There are plenty of scenes of shirtless actors from the period. Of any group, they would have been most likely to "manscape"; and yet, there are none that I can think of. It may not even have been possible without the perfect precision of modern clippers.) They might shave it if they were an actor or vain or obsessed with cleanliness; more likely they just left it natural. Seeing the obvious trim job on Aidan Turner's very hairy - and very obviously gym-toned - chest takes those scenes right out of the period.*
Overall, I thought this version did not give the audience a feeling of being in 1939 but rather in some odd period where the characters acted like modern people, dressed in a sort of blend of past and present, but lacked modern communication devices.
Another thing that seemed artificial was the early emphasis on the date. Several times, we are told that it's August, 1939. Not July, not May or March. August. The degree to which this is highlighted leads one to expect it to matter. What was the point of making a big deal about these events happening only weeks before the start of the Second World War if it's not relevant to the plot? In fact, the international situation is barely referred to at all. The portentous "August, 1939" was a red herring that was irrelevant to the actual story, which seems like bad writing to me.