Here are the problems with Reddit Ads off the top of my head:
1) either all reddit or one subreddit. 1a) no easy way to advertise on multiple subreddits 2) no way to be an agency. So painful to manage multiple accounts even with RES 3) no API 4) no support 5) limited ad units
In short, I would love to advertise on r/SysAdmin but I can't justify spending $20 a day for <100 clicks. Give me the ability to advertise to all of Reddit or specific subreddits. Seriously, why can't I just advertise on a set of subreddits? They should give it to me for the same price as all of reddit. Why? Because that gives me control of my brand. I don't want to advertise on all of reddit because there are a lot of subreddits I don't want my brand associated with, but if I could bid across multiple subreddits I would use the self-service a lot more.
Admittedly, I haven't looked at the reddit ad units in about a year, and I know they're looking for a softdev to build a new ad platform, but geez, can I get a real ad experience please? Reddit has so many pageviews, let me tap that!
Personally, I think it should be solely by impressions, irrespective of subreddit.