I suggest you test your tool with different font sizes (smaller ones, especially) and different settings first and fix all existing bugs before working on additional features.
Agreed. It's actually very simple to fix that, and I should have thought of making it fix it automatically, but I was working on the assumption that the user would like to tweak the font him/herself. Thats what you get when you playtest your own programs! *doh*

Instead of adding new features on top of old code, I'm actually going for a whole rewrite of the entire thing, which will fix and remove all of those types of glitches.

I don't get it, what is the use of Unicode in bitmap font renderer?
If you have an international game you HAVE to use unicode. The one example that really stands out as a good unicode compatible game is Track Mania Nations. People from all round the world play and it can handle Russian, Chinese, Korean chat text ingame. Using, you guessed it, bitmap fonts.