First, the art: so gorgeous, so lifelike and individual, with such speaking expressions. Albus' crooked face and baggy-eyed, benevolent interest, his straggly hair and professorial tie. Argus—he's wonderful, so strained and watchful and narrow, his eyes both yearning and uncertain. It's an amazing face.
Their story is fascinating. The fact that Argus is the reason Albus takes an interest in Muggles and learns to expand his sphere of concern adds so much depth to both their characters. The horrors of WWI that Argus simultaneously suffers and commits creates a strange bond of inadvertent evil between them, because Albus, too, has collaborated in the same indifference toward humanity. Also, Argus' treatment by his parents, reinforced by the wizarding world's disdain, lays the ground for his later callousness, surrounded by students who take their good fortune for granted. I love his hidden despair and his bluntness, in stark contrast to Albus' eagerness—and may I say it's a pleasure to meet an Albus still innocent in many ways.
Then there are the frequent small, unexpected phrasings that bring them into focus, for example: He is a big man on a small bed, half falling, half sliding off one side and the scent of impending come, which make for a poignant, fumbling encounter that's erotically terse and believable.
And then the ending, perfectly set up by what Albus has learned from and about Argus. Of course he'd give young Tom the indulgence that Argus was denied. But oh, that last line. So bone-chilling and concise. What a thoughtful re-imagining of these familiar characters, beautifully illustrated and unsentimental.