1. Using a "Splash Page"
An example would be a home page with a picture and the words "Enter Site". Your home page should be descriptive to be optimized for Search Engines.
2. A Brochure Site
If you want to be #1, you need to be the best. Keep visitors returning by offering more than just your company information. Reviews, resources and blogs are great ideas.
3. Text as Image
Using images of text makes a site look beautiful to the human eye but confuses the computer. Avoid putting important or all text in image form to ensure optimization.
4. No Titles or Descriptions in the page tags
A website address translate into this (www.yoursite.com/thepage). You are missing out on good exposure to keywords affiliated with your business or website. Make sure you include keywords and phrases in the Page Title/URL Link.
5. Same Title & Description on every page
Again you are missing out on the amount of keyword areas a search engine can "crawl". You should give each page a unique Title and Description and be descriptive.
6. Redesign site but keep old Title and Descriptions
Give the search engines something new to crawl and make sure that the title and descriptions reference the redesigned site.
7. No Local Keywords
You should use local keywords. It is how people narrow their search, so get on board.
8. Company "Branding" in Title
If you are a new website or business, no one knows you exist so they are not looking for you. Use words that people will be searching for.
9. Using Hidden Text, Cloaking and False Doorways
Don't try to fool the search engine, it can get you banned. Instead construct your pages correctly.
10. Bad Linking Choices
Do not link with companies who have bad reputations like spam type sites. Search engines frown on this. Research potential linking neighbors to ensure their quality.