Ah, I see more where you are going now, didn't know that you were looking to remove the categories completely. I agree that the categories can be limiting.
I could see removing categories in their physical form, but keeping a drop down for them as a basis for selecting themes, which would have their own selection box. I don't know the limits of design within your software. Basically you have a drop down selector box and a multi selection box with all the available tags. So above that main selector box you can select your "virtual" category (lets call it a theme that works better) which would auto select one or many of the theme tag selections for you, then the user is free to add or subtract tags.
So lets say I select love as a theme from the drop down box, the tag selector box below then autoselects one tag (lets say love) then the user could add other tags by checking them off from the available tags list within the scrolling tag selector box. This would allow people the simple choice that would be similar to categories but also vastly allow more finite description tagging and completely removes categories at the same time. You could even have the tags list autosort the top 10 used tags (or place the theme tags at the top)
The type to find a tag, that's real annoying. I could probably explain it better visually, but I would need charts, graphs, screen caps and my IDE loaded with a better example of what I mean. (ya I used to develop software interfaces at one point in my life) lol. At least addressing all your older poems without tags will be fairly easy