The users now have an option to not see ads (ie. buy a gold subscription), so they can't really complain about having a single ad slot on the page for non-gold users.
To go further, they could not show the ad slot at all if there isn't a campaign running on the site (ie. don't show 'backfill') which would further increase the value of the slot.
$100k+ a month from that ad slot would be very very easy to achieve, and with minimal sales work (they could outsource it to Doubleclick or another network). Sites like Techcrunch and Gawker make millions from ad revenue with a fraction of the traffic - reddit could hit 6 figures a month with a single unit that isn't even displayed all the time.
It would be a perfect balance of retaining the style of the site along with bringing in revenue
TBH, when CN bought reddit, this is what I thought they would do