It depends on what you mean by “quality”. The conventional definition of “quality” in the IT industry refers to error-free code. So using the conventional definition it would be any page that validates without error.
There are tools available for Quality Assurance testing:
The W3C QA Toolbox – Validators, checkers and other tools for Webmasters and Web Developers
When it comes to quality content, we could test for spelling and grammatical errors as a Quality Control method.
Here’s an online tool that can help with that:
Grammar Check – Free Online Grammar Checker
Hint: Try running the text from your OP to see an example of how it works.
And for SEO quality compliance we can use Google content quality guidelines:
Webmaster Guidelines – Webmaster Tools Help
Of course, there are many more specific quality tests that you can do for quality assurance in regards to other features of your site:
Techniques for Web Content Accessibility Guidelines 1.0
Welcome to the HiSoftware Cynthia Says Portal
Site-Perf.com – Know all about your site performance
IDI Web Accessibility Checker : Web Accessibility Checker
Web Page Analyzer – free website optimization tool website speed test check website performance report from web site optimization
You can view quality as subjective, however, I don’t think Google views quality as a totally subjective term.
For years anti-spam algorithms have used linguistic analysis to detect spam and Google has studied similar techniques for detecting low-quality web content. There are a plethora of linguistic rules that could be used to score your content quality.
I have recently noticed significant changes in rankings by making a few stylistic changes of the same content. This was not noticed before the Panda series of updates.
I believe you guys are confusing the term “quality” with “value” or “usefulness”. Those are different words with different meanings. “Quality” implies freedom from error, defect or meeting a standard, not “value” or “usefulness”. I am speaking in context here, the context of search engine listings.