I think what it is is that we associate the old design with good times. We associate it with those deep, late night discussions about something esoteric from back when reddit was a smaller but more knowledgable community. Like any social site, as it grows the average quality decreases. Today reddit is the #10 most popular site worldwide, and largely the discussions are bandwagony / low effort. For me when I see the site under the new design, I don't see any of the nostalgia I have associated with it from a decade of use. I just see all the stuff I dislike about what the site has become.