In Google webmaster central, a query was posed. http://www.google.com/support/forum/p/Webmasters/thread?tid=10cb2f169d48193b&hl=en
This person's site was not been completely indexed even though the site owner claimed that fresh and original content was been published regularly.
Some responses attributed this to a couple of reasons:
1) W3C Markup Validation: The site in question failed the validation and returned many errors.
Improper code may hamper Google's ability to crawl your site. Thus, it makes sense to cleanup the code. But, unless, a site is has too many grave errors, I do not think it should stop spiders from crawling the sites.
The lesson to take back home: Use proper code and validate your web page against w3c markup validation tool. Make sure there are no dead links.
If you rely on SEO for your traffic then do validate your website's HTML code.
2) Google hates Affiliate sites: A contributor reasoned that the site's problem is it is an affiliate site and Google doesn't like them.
This answer can stir up controversy as it rightly should. There are numerous hard working and successful affiliates who will take umbrage to charge.
As one other contributor countered this answer with Google's own views on affiliate sites in which Google has encouraged affiliate site owners to create original content that adds value to site visitors. Google further added that the product category should be carefully chosen according to the targeted customers. The more targeted the affiliate product is to the site users the better chances it will have to good rankings.
It seems that rather then punishing affiliate sites, Google is game for good quality affiliate sites.
What has to be understood is that, Google hates thin affiliate sites and this site which Google perhaps saw as one was bound to suffer. Thin affiliate sites are those with content copied from the parent company's site and providing very little of it's own content or value. All these sites are interested in is to send users to parent affiliate sites. It is obvious that Google won't see any value enhancement in this site and will be hard pressed to give such sites any brownie points.
There must be thousands of other such affiliate sites with similar content. Google won't see any value in yet another similar site and that too which is a part of an affiliate network.
One category of affiliate sites which though may be problematic is MLM sites. These sites are almost universally blacklisted and have a very bad reputation. Sites dealing with MLM do not stand very good chance of ranking well in Google.
Only SEO won't take affiliate sites too far unless they have exceptional original content and incredible value. Without this, it will be an uphill task.