1978 Topps Glenn Burke. High five!
Seriously, however, I can't answer this since I in fact
did quit collecting several years ago. I can't isolate the very card that turned me off. It was the plethora of shiny stuff (Stadium Club, Fleer Ultra) that finally made me stop. By that time, classic issues were getting too pricey for my limited budget, so the main focus was changed to another aspect of the hobby. I did keep the collection (both vintage and modern), but couldn't tell you the last time I looked at an unsigned card.