Having purchased more than a few ads, I can tell you that their platform needs serious help. They're already a step behind because users don't give location or age. To an advertiser who knows the customer, this results in blanketing people you know aren't interested.
The real problem is that targeting ads to interested audiences results in very few interactions. When you target to a sub, you have a good chance of showing up on that sub's home page - but no one really goes to a sub home. You need to get people on their front pages. The front page is hyper-competitive: you are bidding against people who are willing to serve ads to ANYONE (ahem, Hipmunk). I've still had some success, but it varies a lot depending on when people buy ads. Their planning tool cannot be relied upon.
If reddit were a grown-up advertising company, they'd run it on a cost-per-click bidding basis for link/comment/share/save interactions. Votes would be free to the advertiser, but they'd use recent upvotes and downvotes as a quality score to push ads that people hate off the site and encourage ads people don't mind by giving them a competitive edge. They'd also create more sophisticated targeting options, e.g. people who subscribe to /r/A AND /r/B. Or people who subscribe to /r/A AND either /r/C OR /r/D.