I'm fascinated by how many people found their way into Snarry through mentor!Snape. Sort of easing into the idea of the pairing. And my first slash pairing in HP was also Snupin! Well, to be fair, it was past SS/HP and current Snupin. But it really looked like it was headed toward a re-kindling of the Snarry relationship (or perhaps even a threesome!). Sadly, the fic is a WIP that hasn't been updated since 2003.
But, wow. I wonder how many others came into the pairing through AYLNO? Especially people like you who had never heard of either fanfic or slash.