Hidden Text: Text that is either too small or is made to be the same color as the background color.

Pages with off-topic or an excessive amount of keywords.

Duplication of content, either by excessive submission of the same page, submitting the same pages from many domains, or submitting the same page content from multiple hosts.

Word Spamming: Repeating a word over and over over over over over over over over.

No Actual Text On The Page: A page comprised of just graphic images with none or very little text.

Web Site Loads Very Slow: A slow loading page may appear to be "down or broken" and excluded.

Web Site Was Not Reachable By The Crawler/Spider: Sites must be available at the time the engine spiders it or it will be excluded.

Cloaking Was Detected On Your Web Page: Sites that are "cloaked" are often times rejected by engines.

Site Is Under Construction: Some engines do not want sites that are incomplete.

Splash or Doorway pages, or machine-generated pages with minimal or no content with the sole purpose being to get a user to click to another page. Pages that contain Auto Refresh codes are sometimes excluded.

Pages that contain AutoRefresh codes or Redirects, or a DNS that instantly points the user to another domain location are sometimes excluded.

Dynamic Pages often block Web crawlers. Giving your visitors unique experiences, tailored to their needs, is nice but the techniques you use to do that could stop search engines from indexing your content and therefore could greatly reduce your potential traffic. Dynamically generated pages are created on the fly from a variety of elements held in databases. Usually this type of page has a question mark (?) in the URL. When a search engine crawler arrives at that type of page, it captures the content but then halts immediately, and will not follow the links, because it sees ahead of it an unknown infinite number of pages -- a black hole that would trap the server and bring it to a crash.

Pages that have these symbols in the URL: ampersand (&), percent sign (%), equals sign (=), dollar sign ($) or question mark (?). Many spiders do not recognize them.

Pages that contain only links to other pages or domains.

Active Server Pages, pages that end with (.asp) that have question marks in their URLs (indicating that the page is a script for the construction of a page, rather than just static content) are most often not indexed.

Hate Sites or Illegal Content: Sites that profess hate or illegal activities are sometimes excluded.

Be Careful Who Your Neighbors Are: Be careful about where you have your pages hosted. If the hosting service also hosts spammers and pornographers, you could wind up being penalized or excluded simply because the underlying IP address for that service is the same for all the virtual domains it includes. You see, when search engines detect a "search engine spammer" or other websites that they want to exclude from their search engine, they can and often do "BLOCK" ALL submissions from that particular IP Address or Hosting Machine. This means that if your web site is on the same IP address or same Hosting Machine as the "blocked" website, your site may also very well be blocked from that engine. If your having problems getting your web site listed on many of the engines. You may have been blocked due to your neighbors. You may want to request that your Hosting company move your site to another machine or IP address.