There are some nice minor parts for familiar faces (Colin Firth, Benedict Cumberbatch, Mark Strong, Andrew Scott), but mostly it's carried by these two young actors I'd never seen before slipping past the German forces to get a vital message to a British contingent. They certainly did an able job - affecting action but not hammy. In a lot of ways it reminded me of Frodo's and Sam's journey through Mordor in ROTK. And the characters had real differences and growed as the movie progressed in a way that made sense and kept me involved.
Its a war movie about trench warfare, so yes, obviously there's violence and gore. But it's not overly explicit. I'd compare it to the sequence in the third Hunger Games movie where Katniss returns to District Twelve after it was firebombed. The film's rated R, but I wonder if it couldn't have earned a PG13 rating with a little less swearing. If it was (say) a YA dystopia rather than a war movie marketed to adults. It didn't strike me as too far over that line.
At the moment I'm thinking about what it says about violence vs peacemaking and individual choice vs the forces that keep the masses pointed in the same direction. If you know much about WWI films, you're probably familiar with the trenches and the vast armies moving against each other in waves - there's just not the geography to allow them space to do anything other than carry on. That's different for the two characters at the heart of this, and for me one of the most moving scenes was easily when one of these two soldiers at the center of this movie is running across a battlefield, perpendicular to the charging line, no gun or pack but just so desperate to get his orders to the commanding corporal. It's an act of individuality and freedom set against the seemingly-unstoppable rush of the battallions as they run into battle. But it's freedom he only has because he's been ordered to leave his own regiment. This isn't a mission he ever wants! So it's a really interesting take on freedom and the singular hero versus the bulk of the army. It was beautiful and thought-provoking, though I don't really have any answers.
I was sure it had been overhyped, and to a certain extent it was, but it was also one of the quieter, most thoughtful, most character-driven war movies I think I've ever seen.