One thing I disliked...
...were the cameo appearances popping up for a couple of minutes during the film. "Oh, it's Colin Firth; Look, it's Mark Strong! Now here's Benedict Cumberbatch!" When Cumberbatch appeared, there were audible gasps, whispers and even chuckles from various points around the theatre.
1917 is a very good movie, but lesser-known character actors should have populated these tiny but key roles, I'd say.