In one of the first episodes I saw, O'Brien looked in the mirror and said to herself This is not who you are. That struck me as very much out of place for the time, ca. 1914. It's definitely a contemporary American thing, and I suppose a British one, too. I kept noticing things like that--characters using idioms that seem far too late-20th/early-21st-century to seem authentic for either rich or poor English people of 1910-20: for instance, Daisy following some remark with "I'm just sayin'." Sunday night, just for fun, I kept a list.

Last time I checked  Used ironically to introduce an obvious or indisputable fact, as in "Last time I checked, the sky was blue." (Come to think of it, the character may have said "Last time I looked," but it was the same idea.)

Get on with her life  Need I say more about this pop-psych tic that ranks with "closure" for the most over-used way of describing the need or wish to put the past behind one?

That's not who we/I/you are/am  Ditto, more or less. It's appeared several times since I first noticed it. I recall hearing this in the '70s, and I suppose it may have been around for a time before then, but surely the whole self-regarding real-me stuff comes from the late 20th century therapeutic mindset.

suck up  I think it's very possible that I could be wrong about this one, but surely even if it were around 100 years ago, would a wealthy young lady have openly announced her intention to "suck up to my mother-in-law"?

I don't think this is a good idea  Not the words themselves, but their ironic use, meaning "I think this is a mistake."

How does that work? Again, not the words themselves, but their context and use: not a simple question about, say, an automobile engine, but an odd present-tense way of pointing out an impending obstacle, more or less equivalent to "That's not feasible," as in "Ride a bicycle in this traffic? How does that work?"

Figuring I'm not the only person to be bothered by these, I Googled "Downton Abbey anachronisms," and sure enough, I'm not, at all. Be careful about any sort of query into the series, though: because season 2 has already shown in Britain, it's easy to stumble onto spoilers. Not that I care very much about that at this point.