This movie is not for everyone. Here's why I liked it: I already love the work of many of the actors involved. This was an odd project (it feels vintage and not due to costuming) and I get the reviewers grossed out by the "falling in love bit", but I thought it very well done. It's intended to be mildly unsettling/disturbing in the way some scenes were shot. Some of Judy's staring was odd and a bit discomfiting until you realized how inexperienced she really was. After a bit I realized she probably wasn't even aware of how that came across. That said, men have fallen for far younger women since the dawn of time.
This is a story told through subtleties and most of the actors reactions/behaviors are flawlessly portrayed.
Judi Dench was introduced as a childlike character, reliant on and submissive to her sister in a giddy, open manner which suggested she'd spent her years unspoiled. I believe we were looking at a naïve, sweet old virgin. She clearly didn't understand her feelings initially and lacked the sophistication to hide her rapidly growing obsession. Several of her expressions (her sparkly, smiling skip and her head lowered, eyes raised gaze through the window) showed further how much more she remained a freespirited girl than the old lady on the outside. Her sometimes petulant reactions and her jealousy all point to her emotional inexperience.
Maggie Smith was the responsible one, she sheltered her sister and protected her innocence as she could, supporting her sister's joys tempered externally with reality. She'd been married and lost her husband in the war - a war in which she'd served as a nurse and learned to speak some German.
Maggie's better ability to communicate with Andrea caused her sister additional angst. She was excellent in portraying her growing, reserved fondness for the young foreigner, and when she began to show a maternal jealousy nearly as strong as her sister's, it was charming.
When Judi cries, speaking to her sister about her sister's husband, we see a deeper side; she senses the magnitude of what she's missed out on and though she realizes she can't have it with Andrea now, she can't stifle her feelings for him and true to character, doesn't try to, she just keeps the romantic bits a bit more hidden.
I wasn't impressed by the way Andrea left as he did. After all they'd done for him it seemed ungrateful (he could have taken another ship later) but in a way that was kinder, as it allowed Judi to let her romantic notions for him go (when she let his hair clipping fall away into the wind).
So yes, it's subtle and may feel plodding for some, but I really enjoyed the humanity of the realistic reactions, especially of the two leading ladies.