Ironically, content discovery.
Reddit's biggest problem is that the big subreddits are incredibly homogenous, and the small ones incredibly slow. If you want to read good, in-depth analysis of world affairs, then you'd better agree with the popular sentiment on /r/worldnews or /r/truereddit. If you don't, you're SOL.
Often, the good content you find is in the comments. However, this necessitates digging through thousands of comments that are generally low-content. This causes pain.
Want to solve a problem redditors have? Find a way to separate the wheat from the chaff.