"Good" is still going to be a subjective term in most regards, but here's how I see it...
I have a general checklist to refer to kinda like Trev said. Fast loading pages, lots of unique content, easy to navigate, etc. All the stuff that makes a site useful and usable. You could even go through that Google document leaked earlier this year (their updated manual review guidelines) to create a checklist. Really, what better checklist than Google's own?!
Then each item would have sub-items based on the site's topic. For example, sometimes good content only requires highly useful articles, but sometimes it means infographics, expert interviews, how to articles, resource lists, calculators, tools, etc.
Also, take a look at sites you absolutely love. Perhaps Amazon.com, but also the top sites in whatever niche you're targeting. (If you don't know the niche, scrape some related forums and see what sites people link to frequently.) Then make a list of everything you like about those sites. Make sure your niche site has all the features of the best sites in that niche, then try to add even more features based on top sites you've seen in other niches.
