Actually, reddit is NOT one of the most profitable sites on the web at all.
In fact, we are not profitable.
This is because increased traffic results in more server usage, which primarily increases costs, not revenue. In theory advertising revenue should/could scale with traffic, but since we never tried very hard to sell our advertising inventory, we only run ads on a relatively small percentage of our pages and they do not cover our costs. When Obama comes to "chill on the weekends," that increases costs, not revenue.
It is true that given our massive pageview count, we could theoretically load up our pages with ads and probably make enough to cover our costs. However, that would significantly degrade the experience of using the site. Before joining this company, I was a redditor too, and if reddit had done this I probably would have stopped using the site.
Because the site is not profitable, we have a choice to make about how to cover the increasing costs of our skyrocketing traffic. We can run a lot more ads (interestingly, the spammiest and most annoying ads pay the most) or we can create reasons for users to pay for the site.
See, the problem is that if your site is funded primarily with advertising, then you are beholden to your advertisers. If your users choose to post something politically or culturally controversial, you come under editorial pressure from advertisers to remove or modify it, because advertisers like bland, well-lit spaces. This eventually results in a watering down of the true, authentic content on the site (remember Sears?). It's one of the reasons Digg failed. And personally, I feel that's not the best way to serve the community. It's not the right thing to do for the users who have faithfully contributed to reddit all these years.
Rather, we should be beholden to our users. That is, if most of the money is coming from users, then we'll answer to the users. So this means that yes, we are asking you for money. If you choose to pay us, if you're the ones keeping us afloat, then when you yell at us and want us to do something, we'll do it. THAT is why we're promoting reddit gold, and that is why the reason we're doing this is not just to make money to cover the costs, but to do so in a way that benefits the community.