1. Using the wrong keywords
The first thing you should do is keyword research. If you get this wrong your off to a non starter. Make sure you target the best keywords for your niche, ones that searchers actually are using. For example, if you have a site that helps people save money, think about keywords searchers will type in such as voucher codes, or money off.
2. Forgetting to use title tags
This is a common one often overlooked by beginners. Title tags are important and must be different on every page of your website.
3. Over doing flash
Flash cannot be read by googles bots. If you are going to use flash, it is essential that you also use the html mark-up for it.
4. Back link spamming
This is something even the best webmasters are failing to grasp. Link farms are not a good thing with Google. You will get much further by getting high quality one way links from website with high pr.
5. Keyword stuffing
Your content must make sense. If you try and over do the keywords you will get penalized by Google quickly. What you write must flow.
6. Keyword density must stay below 5%
7. Duplicate content. Google loves fresh, unique content. Google bots are very clever at spotting content that is used elsewhere.
8. Inadequate navigation
Your website must contain no broken links and have good navigation. Both visitors and search engine bots need to get around your website.
9. Missing the value of headers
No harm in using images for headers but headers are important and you need to make use of them. Search engine bots don't read images and headers are the first thing they read, surely it makes sense to include your keywords in your headers?
10. Unreliable web hosting. Cheap may please your wallet but it will annoy your visitors and stop you getting high search engine ranking. Google will think your website is not there if it happens to crawl your site during an outage!