I have a really hard time finding manga and would like a feature that gives you a compatibility score between your reading list (or other lists) and someone else's reading list. I feel like this would be the best way to find new manga, much better than a score/rating, genre list, and/or category list because it's likely that someone who has read 95% of the manga you've read will have lots of things you like in their 5% of manga you haven't read.
It could be more complicated, taking into account scores and what-not, but the basic idea would be like this:
My reading list: naruto, bleach, one piece
Person A's reading list: naruto, bleach, shokugeki no soma Person B's reading list: naruto, bleach, one piece, fairy tail Person C's reading list: skip beat, shinigamihime no saikon, chihayafuru
My list of compatible manga list would have: B (100%), A (66%), C (0%) and then I can click their lists and see what they've read that I haven't with the current interface of check marks/list icon/etc.
Obviously only done with public lists, they could even be anonymous lists but it would be a great help in finding new manga.
If you go to a manga's page and click on one of the hyperlinked numbers under "List Stats", it'll give you a list of users and how many manga you both have on your lists. For example, if I go to the Naruto page and click on the 13722 in "on 13722 reading lists", I get a pop up telling me a long list of members, and I can look down and find someone with a high number of intersecting list items or a high percentage.