Good design is
functionality: whatever it is, it has to do its job
neatly and quickly
innovation: good design has in some way to be an improvement
over previous designs. new ideas, new solutions to common
problems that go beyond just new visuals
ergonomics: if it's a thing, the user should be able to
use it without strain, serious injury, complicated thinking
and other nasty things.
elegance: good design is pleasing to look at and touch. kinda good design is complying with current trends. like that one day when every computer thing started to be white and green. really good design is timeless. like these things from 40 years ago that manage to still be impressive nowadays.
simplicity: good design should be straightforward and easy to use
so much for sweet talking. Here come the 'real-world-additions':
Good design is
low manufacturing costs: you can't just make it for cool's sake. if your company busts because of it, good design doesn't make it beyond prototype stage.
appealing to the masses (which means, nothing too fancy): good design is NOT necessarily elitist design. something less visionary that benefits many people may be better design than something ultra-cool-fancy-geeky that only some can/want to use.
environmentally friendly: if it appeals to the masses, it has to be produced with the environment in mind, otherwise there will be no more masses to sell good designs to sometime soon. made of recycled materials, longtime usability, low energy consumption and low emissions (if applicable) as well as recyclability, once the good design thing has done its duty.
and there's more to come. feel free to add...