Having a list of tags already in use, and possibly having a small popup that suggests tags that already exist, would be good. It could potentially reduce the number of similar tags over all, and can also help with filtering. In addition, allowing users to exclude certain tags from searches would also be good, and would allow for a more fine-tuned search feature. Rather than having the broad NSFW-Graphic-Violence (GV) or NSFW-Porn tags, that will result in all stories marked with them being filtered out, this would allow people to search within their comfort zone. For example, since any amount of GV should result in a story being marked as NSFW (even a single scene) it would also require the story to be marked with the GV tag; meaning that, if you are OK with some GV, you would have to search without having the GV tag filtered out. Expanding the usefulness of user tags would allow you to search for light GV, or soft porn, while excluding the rest.
Under consideration feature