Splash Page

I've seen this mistake many times where people put up just a big banner image and a link "Click here to enter" on their homepage. The worst case -- the "enter" link is embedded in the Flash object, which makes it impossible for the spiders to follow the link.

This is fine if you don't care about what a search engine knows about your site; otherwise, you're making a BIG mistake. Your homepage is probably your website's highest ranking page and gets crawled frequently by web spiders. Your internal pages will not appear in the search engine index without the proper linking structure to internal pages for the spider to follow.

Your homepage should include (at minimum) target keywords and links to important pages.

Non-spiderable Flash Menus

Many designers make this mistake by using Flash menus such as those fade-in and animated menus. They might look cool to you but they can't be seen by the search engines; and thus the links in the Flash menu will not be followed.

Image and Flash Content

Web spiders are like a text-based browser, they can't read the text embedded in the graphic image or Flash. Most designers make this mistake by embedding the important content (such as target keywords) in Flash and image.

Overuse of Ajax

A lot of developers are trying to impress their visitor by implementing massive Ajax features (particularly for navigation purposes), but did you know that it is a big SEO mistake? Because, ajax content is loaded dynamically, so it is not spiderable or indexable by search engines.

Another disadvantage of Ajax -- since the address URL doesn't reload, your visitor can not send the current page to their friends.

Versioning of Theme Design

For some reason, some designers love to version their theme design into sub level folders (i.e. domain.com/v2

Exceeding the 65 character limit:
Many bloggers write very long post titles. So what? In search engine result pages, your title tag is used as the link heading. You have about 65 characters (including spaces) to get your message across or risk it getting cutoff.

Keyword stuffing the title:
Another common mistake people tend to make is overfilling the title tag with keywords. Saying the same thing 3 times doesn't make you more relevant. Keyword stuffing in the Title Tag is looked at as search engine spam (not good). But it might be smart to repeat the same word in different ways:

"Photo Tips & Photography Techniques for Great Pictures" "Photo" and "Photography" are the same word repeated twice but in different ways because your audience might use either one when performing a search query.

Empty Image Alt Attribute

You should always describe your image in the alt attribute. The alt attribute is what describes your image to a blind web user. Guess what? Search engines can't see images so your alt attribute is a factor in illustrating what your page is relevant for.

Hint: Properly describing your images can help your ranking in the image search results. For example, Google image search brings me hundreds of referrals everyday for the search terms "abstract" and "dj".

Unfriendly URLs

Most blog or CMS platforms have a friendly URL feature built-in, however, not every blogger is taking advantage of this. Friendly URL's are good for both your human audience and the search engines. The URL is also an important spot where your keywords should appear.

Example of Friendly URL: domain.com/page-title