There's two usability limitations of iOS app that could easily be fixed
1) The font is just too small. On my iPad Mini, it's tiny and there's no way to change this. Why not let people use custom fonts ?
2) The Notes app needs quick buttons for bold, italic etc., and it needs to render them in edit view just like it does in preview. Not too many people like to type < b>< u> Title< /b>< /i>. This could easily be made transparent to user.