A good 80 is probably a consistent 80. An 80 that hangs together. An 80 that delivers benefit. An 80 that doesn't just happen to be a collection of the bits of the right thing that it's hoped will take 20 to build.
When you're working on an 80/20 project, also look for an 80 that won't detract from what's already there and render the result of the 20 no more than a non-eventy.