The biggest problem with Reddit advertising currently is the pricing. Even if you're advertising on a large subreddit - the only way to target your ads, really

The biggest problem with Reddit advertising currently is the pricing. Even if you're advertising on a large subreddit - the only way to target your ads, really - it tends to work out rather expensive for what you're getting. Obviously some people are making it work, and it's probably cheaper on the really large subreddits. The reason for this is their odd "pay per day" system, which has a $30 minimum. However, they're moving to a CPM system soon, I'm told, which will make Reddit a very interesting ad opportunity.