Definitely the fact that I can watch far more Netflix in Classic ... just had no time in Retail.
Between the hours spent walking around and the hours spent waiting for a spawn and the hours spent killing boars until you find one with a heart - you get to go through whole seasons of Netflix shows.
Also - as an anti social person, I love how Classic discourages grouping up by ensuring that that rare as hell boar heart only drops for one person in the group.
Also love how I don't get to do dungeons - doing dungeons in Retail was so painful, but in Classic they aren't an option.