Terrific, terrific Slughorn. Not a caricature, not a repulsive idiot. I love his various self-assessments, his pauses, his private admissions.
I'm having the urge to quote lines back to you, only I'm not sure what I want to say about them. There were moments that totally arrested my attention, not in a distracting way, but because they so perfectly capture a certain sensibility. The initial description of Benjamin/Severus. The way Slughorn notices, and had noticed in the past and remembered, tiny details like the boots. This characterization just gah! Words fail. I love that he's something of a mental dandy, because of course he would be. I'm totally incoherent. Um, I'll quote back one line, because it was so perfect, so understated, so oblique and yet so clear: "Oh, I never." He paused. "Not since Albus took over, at least." Did I say wow, before?
You did an incredible job of sowing doubt about whether Benjamin really was Snape. That's just amazing to me, because yes, Snape fest, so of course it's Snape, and yet while reading, I thought it entirely possible that it would turn out not to be Snape, and the fic would still totally fit the pairing. I love that Slughorn manages to cast Snape's every snotty utterance into a positive light. Really, there was nothing I didn't love. Your writing is amazing.