I know I disliked reddit's design when I saw it the first time, and I've spoken to several people who never got into reddit because they disliked the design.
This large group of people, probably the majority, does not want to give such platforms a chance, and they do not want to get under the covers and figure it out. They simply do not want anything to do with something that is tightly-packed or primarily text-based. Many of the people I tried to recruit to reddit in the early days (either reddit.com or sites running the reddit software) had this reaction.
This can be a feature when you're trying to target specific types of people (cf. HN). But if you're trying to sell ads, it's a bug, because ad sales are all about getting as many human eyeballs on your platform as possible. Perceived ugliness is a real barrier to mass adoption.