Why would there be a dedicated tag for Ecchi? Once people start doing that, you are down the road to meaningless, one-time use hash tags that offer no value and actually decrease people's ability to filter for topics.

I get that there should be an option to filter out sexual and violent content independently of each other, but those filters should be very broad categories and not serve as organizational mechanism for the individual preferences of users.

Also, if you introduce a rating system, who will handle the additional moderation load, checking out in which category each particular content instance falls. I don't see it.