It will be really useful for designers to have a convenient way to test OpenType font features, something that no free software font viewer offers yet (at least none I'm aware of). Right now you need to use otfinfo from command line to know which OpenType tables are available on a particular font and then test how those characteristics look like either by using XeTeX or LibreOffice Writer 5.3+ (it got OpenType support!). Adding the capability to apply different font features to a sample text will make kfontviewer a really useful application.
I think this missed feature it's a "must have" one today... Please, consider add it soon... All of we who needs to work with fonts (from graphic dessigner to technical writters crosing the wide spectre of others specialities) Thanks in advance... E.