Agreed.
However, I think the tags that will be used should be agreed upon and listed first.
Maybe a system similar to Hulu's tagging (tags others have applied are suggested to users in order to evaluate videos) using common themes for manga would work well.
Examples:
-type of sex depicted (#straight,#yuri,#futa, #incest, #rape, etc)
-themes (#school, #office, #countryside, #cosplay, #uniform, #bondage, #magic, etc)
-genre (#romantic, #tsundere, #yandere, #ntr, #fanwork/#doujin, #original, etc)
It could probably work the most effectively if there are set standard tags. :)