I don't think it's some kind of technical barrier that prevents 'non-techies' from using reddit & imgur, it's the incredibly toxic culture
Now that's not to say that once people do figure out how to navigate, they're turned off by the culture. However, I do think the UI has something to do with more people not joining.
Personally, I feel even 4chan is much cleaner than reddit (yes I know 4chan uses linear threads while reddit supports branching. Hence the HN example)
I guess if one stays on the main roads and doesn't stumble into the dark alleys, he'll be ok.