Something I often run into is the trouble that I cannot filter out characters with certain negative tags, as they are ambiguous with simply not having that tag. I also cannot filter out characters who are Idle, or who are Away; although the color of their name is helpful, it can sometimes double the length of the search result, cluttered with people who may or may not even be at their computer.
I think it is incredibly important to be able to filter out people who have declared a hard pass for certain things; this is a strong contraindication for compatibility between roleplayers. Stronger, I would argue, than having certain tags in common.
Filtering characters based on activity status is just handy. I could live without it, but it’d be nice.