There is an excellent thread at WebmasterWorld discussing "25 signals of poor quality sites" that is a good read. While I don't agree with all of the points that are being made, and many are simply mistakes made by novices, it's definitely a good refresher read and something to share with those just getting started on their path to Web success.

A few of the signals that I tend to agree with most and that that I use with the greatest frequency in determing the quality of a site are the presence/absence of link request pages, presence/absence of a privacy policy, availability of contact information, amount of unrelated subject information, and the presence (or lack thereof) of on site errors - Php/MySql or just 404 pages.

Read the entire thread at WebmasterWorld.