The art is gorgeous (o, not-so-mysterious artist!) - their wonderful hair, with Severus' so straggly and Firenze's funny little braid, not to mention his wonderful tail, and I love the little awkwardnesses of their body shapes as they try to fit each other. The positioning of Firenze's hands in the second picture is so perfectly hot, and I love Severus' delicate fingers and pink knuckles.
As for the story, it just fills me with comfort and glee, what with the Pony Club and Severus pissing in the great 'outdoors' and Severus' grandfather and munching on fiddle head ferns and especially (especially!) Firenze murmuring, "Oh, you darling." Just, everything.