I don't think mainstream advertising that appeals to the general public will ever be successful on Reddit. Niche advertising, such as recruiting and product placement in subs like /r/sysadmin, or fanduel in /r/nfl, will have mild success - but not enough to make Reddit profitable. Ironically I think imgur will have more success then Reddit in the long run.
Mainstream advertising doesn't appeal to the general public. People tolerate it because they don't know they can turn it off.
If web browsers shipped with ad-blocking turned on by default then most people would never turn it off.