I would simply like to see Opentype support. Unless there's a plugin that I'm not aware of I can't use any of my purchased fonts for design work. Which I'm sorry to say renders the current iteration of Paint.NET fairly useless to me unfortunately.
Although I must say that I am quite enjoying what has been achieved in the program thus far from my limited experience.
Is .otf compatibility really such a big hurdle for the app?... or, is it included on the roadmap for a future release?