depicts... without actually depicting... You don't see anything on screen... And there's not a lot of exposition either

So, "Neither showing nor telling in 'M' dir. Fritz Lang"? Seriously, take the adage "show, don't tell" (or any film prescription for that matter) with a pinch of salt, because there're many good films that for a variety of reasons tell and not show, eg. Diary of a Country Priest (1951), The Satin Slipper (1985), The Last Letter (2002)...

the viewer to fill in the blanks with their own imagined horrors, most likely more terrible than what Lang would show if he could

Have you ever visited bestgore.com or the like? Just asking. Don't do it if you haven't.

What really struck me...

Folks who think contemporary films are mostly inferior to old films commonly make the mistake of hasty generalization. Like stacking current run-of-the-mill movies against a cream of the old crop like M... do you think that's legit? Old films also have their abundant share of turds, and Lang himself made some crap as well — check out his Dr. Mabuse the Gambler, Woman in the Moon, and Indian Epic. If you think spending half an hour on exposition is bad, wait til you see a three-hour exposition! So I say compare apple to apple, and turd to turd.

IMO Lang made his best films when he worked in Hollywood. My fav among these is The Big Heat.