Make a UI change that makes it more clear when you've accidentally made one mega-tag.
When someone accidentally tags a topic without using commas they end up with one mega-tag for the topic. We could give each tag in the tag list a background color to show a visual separation. This would make it more obvious that all of the tags are merged together.
Example solution: https://i.imgur.com/9COWYB6.png
Edit: If you like the suggestion I'd be interested in doing a MR. Just for the sake of saying I got a MR on Tildes.