Let's try a little exercise...

Common features of spam domains include:

• Long domain names
• .info, .cc, .us and other cheap, easy to grab TLDs
• Short registration period (1 year, maybe 2)
• High ratio of ad blocks to content
• Javascript redirects from initial landing pages
• Use of common, high-commercial value spam keywords like "mortgage," "poker," "texas hold 'em," "porn," "student credit cards," and related terms
• Many links to other low quality, spam sites
• Few links to high quality, trusted sites
• High keyword frequencies and keyword densities
• Small amounts of unique content
• Very few direct visits
• Very few links sent out in (non-spam) email to the site
• Registered to people/entities not associated with trusted sites
• Not frequently registered with services like Yahoo! Site Explorer, Google Webmaster Central or Live Webmaster Tools
• Rarely have short, high value domain names
• Often contain many keyword-stuffed subdomains
• More likely to have longer domain names
• More likely to contain multiple hyphens in the domain name
• Less likely to have links from trusted sources
• Less likely to have SSL Security certificates
• Less likely to be in directories like DMOZ, Yahoo!, Librarian's Internet Index, etc.
• Unlikely to have any significant quantity of branded searches
• Unlikely to be bookmarked in services like My Yahoo!, Del.icio.us, Faves.com, etc.
• Unlikely to get featured in social voting sites like Digg, Reddit, Yahoo! Buzz, StumbleUpon,  etc.
• Unlikely to have channels on YouTube, communities on Facebook or links from Wikipedia
• Unlikely to be mentioned on major news sites (either with or without link attribution)
• Unlikely to register with Google/Yahoo!/MSN Local Services
• Unlikely to have a legitimate physical address/phone number on the website 
• Likely to have the domain associated with emails on blacklists
• Often contain a large number of snippets of "duplicate" content found elsewhere on the web
• Unlikely to contain unique content in the form of PDFs, PPTs, XLSs, DOCs, etc.
• Frequently feature commercially focused content
• Many levels of links away from highly trusted websites
• Rarely contain privacy policy and copyright notice pages
• Rarely listed in Better Business Bureau's Online Directory
• Rarely contains high grade level text content (as measured by metrics like Fleisch-Kincaid Reading Level)
• Rarely have small snippets of text quoted on other websites and pages
• Cloaking based on user-agent or IP address is common
• Rarely contain paid analytics tracking software
• Rarely have online or offline marketing campaigns
• Rarely have affiliate link programs pointing to them
• Less likely to have .com or .org extensions
• Almost never have .mil, .edu or .gov extensions
• Rarely have links from domains with .edu or .gov extensions
• Almost never have links from domains with .mil extensions
• Rarely receive high quantities of monthly visits
• Rarely have visits lasting longer than 30 seconds
• Rarely have visitors bookmarking their domains in the browser
• Unlikely to buy significant quantities of PPC ad traffic
• Rarely have banner ad media buys
• Likely to have links to a significant portion of the sites and pages that link to them
• Extremely unlikely to be mentioned or linked-to in scientific research papers
• Unlikely to use expensive web technologies (Microsoft Server & Coding Products that Require a Licensing Fee)
• Likely to be registered by parties who own a very large number of domains
• Unlikely to attract significant return traffic
• More likely to contain malware, viruses or spyware (or any automated downloads)

For high quality content domains, the opposite is true (at least, for a good percentage of these). Now think about the sites you're building - which features apply to them? What could you do differently to be more like the "high quality" category and less like the "spam"?

BTW - Love to hear your take on features you think are common to spam, or to high quality sites.