1. Hidden text in the name of accessibility
Many web designers are using a CSS technique that hides text – allegedly for accessibility. The technique uses a background-image to replace text. It is a common CSS technique touted as a way to maintain accessibility while still displaying a graphic instead of the text header. Unfortunately, it fails in many screen readers (see above article), and it is considered by the search engines to be hidden text!
2. Unintentionally spamming keywords
Many sites use words in their keywords, title and description that are not ever used on the page. Most webmasters insert keywords into these tags that aren’t used anywhere on the page. The problem occurs when keywords are brainstormed separately from content development. Good SEO involves doing the two techniques together.
Don’t expect Google to just trust that you are relevant for these keywords. How relevant could you be for your keywords if you never use them, anyway?!
3. Excessive Links
Webmasters have become obsessed with their Google Pagerank, and are trading links at an ever furious pace. Having too many outbound links on a single page makes your site look more like a “link farm” than a legitimate website. You should try not to place too many outbound links on a single page.
If you do need to link to 100 or more sites, place the links on separate pages. Instead of focusing on an unhealthy amount of links, create quality content and allow the links to flow in NATURALLY. That is what Google wants to see anyway.
For Google’s webmaster guidelines, visit http://www.google.com/webmasters/guidelines.html
What to do if you are banned? Google’s Matt Cutts has the answer: After you have fixed your site, file a reincluson request.