Re: differences - I would say Reddit is 1) disorganized and 2) has its focus primarily on front-page. I don't know many people who know how or have to time to dig through all the sub-reddits offered to maybe find something they like. This structure does work for many of us, and that's cool. I think it can be improved upon, though.

Same with your argument about untapped markets. I'm sure Hackers had a preferred method of communicating pre-HN (IRC?), but something better and more structured came along and displaced it.

Re: HN business model - paid job board, paid newsletter, hacker conferences, hacker news apparel, regular old google ads, etc. there would be many paths to take if this was a for-profit endeavor.