Lesson learned: Not all traffic is created equal!
I've found this kind of temporary traffic burst from high-profile sites is often very transient traffic, people clicking on a link out of curiosity because they saw it somewhere, with a very high bounce rate. It's the kind of traffic from people who open 30 tabs and then briefly visit each one. Doesn't matter (at least for my sites) whether it's Reddit, Slashdot, HN, an NYT Blog, etc.—almost never has the same CPM as "regular" traffic, regardless of the burst source. Visitors who come via organic search or bookmarks are much more likely to spend more than 1 second on the page and read/interact with something.
20k visitors for the front page of Reddit sounds low, though. I've hit the front page (with http://www.reddit.com/r/dataisbeautiful/comments/2ac8ba/who_... , also a default subreddit) and I received 150k visitors.
On the 20k, it's actually low for Reddit, but it's so volatile and random, i've had posts generating 185k+ views for around the same amount of points in active sub reddits.