In addition to that, the software behind HN doesn't do nearly as much as the reddit software: there is no automatic checking of new messages, there are no sub reddits you need to work on, there are only so many votes to be processed, etc.
I have no clue about who pays for what, but I guess PG pays.
When your userbase is filled with college kids and anti-capitalist, you are going to have a tough time making money,
After all, they're aiming for 2% of the userbase subscribing. That's a large number when they're offering basically nothing of value in return. I know other 'freemium' business get 1-2% subscribers, but surely that's when they're offering a real improvement in the service, such as Dropbox's 2GB -> 20GB.
There is a good argument that advertising actively moves you away from the EMH which is one of the strongest arguments in favor of capitalism.
I said anti-capitalist because many people were against advertisements and the gold accounts. While there may be many people lurking on reddit that are not anti-capitalist, it's a sentiment I see in almost every topic related to making money on that site.
The users of reddit are wide and varied. There is a significant number of redditors who have demonstrated their willingness to pay reddit, me among them, and don't regret it.