There should be no such thing as a default sub for new accounts. Users should pick what they want when they sign up.
the defaults are important because they let people use the site before they decide they want to sign up
Don't road block me with some set of tiles and vague "business, technology, fashion, music" screen.
Reddit has it right that a new account should have some baseline. Their discovery tools however are not intuitive, if you can even call them tools.
have defaults with strict moderation, and alienate newcomers
have defaults with loose moderation, and the voting algorithm will favor the most superficial, bitsized posts.
All the best!