Let's look at some of these supposed usability factors.
1. Conversion score
Irrelevant to many sites - for some a conversion is just someone clicking on an ad or just someone viewing an ad.
2. Bounce rate
I actually think this is good - but it is not someone just visiting your site and clicking away. It has to be looked at as someone clicking a SERP result visiting your site and then clicking another site from the same SERP.
Even that is somewhat floored as many eCommerce sites may be disadvantaged by people who are price comparing the various sites with the same product (but hopefully all results in the same SERP will have a similar level of disadvantage)
3. Number of page views
This is absolutely not an indicator of quality (unless you can very specifically categorise the intent of a site)
If you a news site - getting goodly numbers of page views per visi is good as people are reading more of your articles.
If for instance you happen to be providing lists of ASCII character codes or similar type information then getting a high number of page views would have to be considered really bad as the user who is looking for this information is generally looking for a very specific answer to a very specific query and generally wants to find this information as fast as possible.
eCommerce sites lie somewhere in the middle as they are trying to balance users buying as many products as possible (high page views good) and users finding the products they want as fast as possible and then going through the checkout/cart process as fast as possible (low page views good).
4. Number of repeat visitors to this page
A good indicator of quality perhaps - or perhaps just an indicator that the site has a lot of offline presence (such as a TV station site)
5. How many people add products to cart after visiting this page?
Irrelevant to non-ecommerce sites, but a good indicator for ecommerce sites.
6. Average amount of time that is spend on this page.
As for number of page views this factor depends on the type of site - as per the examples above a news site wants a high time on page - it probably means that the user was interested in the article and read it all. The ASCII code type site would want a fairly low time on page as if the user takes too long in finding the specific information that they want, the page is probably too confusing and the user may have thought 'I've had enough of this I'll try another site'
In my opinion quality is defined by the purpose of the site and how effectively it fulfills that purpose - and no amount of statistical analysis will tell you that.
Trying to apply data driven quality indicators is a bad plan and will lead to a very homogenised web, dull and boring with some queries being answered effectively and others providing totally useless results.
Moving on to other factors that could be deemed a measure of the quality of the site which weren't mentioned earlier
The size of a site could be a factor in quality, especially regarding backlink analysis, as a large site should generate more natural backlinks and they should be of a more diverse nature.
This of course ignores scraper and auto-generated content as these can produce very large low quality sites (as we have seen in the past).
Small sites, such as many EMDs, are often produced to provide quick answers to specific information requests and will generate few and very focused backlinks (think similar anchor text) - this does not mean that they do not answer a very small number of queries extremely effectively.
There are some interesting things in psychology and one of them is the Belbin Team Inventory - [en.wikipedia.org
If you look at these - you can see how these personality types can be applied to webmasters, some will love large projects which they can add to and add to and never complete, others will get satisfaction from completing a job and moving on to the next job. Some will just start a project and get bored - and then start another one without finishing the first.
If you look at people who get satisfaction from completing a project - they are likely to want to produce a small website (for which EMDs are absolutely ideal) which they can 'finish' before moving on to the next project. (Some may return to do a revamp project on their website at a later date, whereas others will just start the project over again on another domain and try and learn from the information they got from the first project because their personality doesn't like updates and upgrades)
Site age can be considered an indicator of quality because sites that are not successful will often be abandoned and the domain recycled, however this is not necessarily true for all domains as there have been Search Engine Spammers for very nearly as long as there have been search engines (probably the day of the first launch of a search engine!).