These are some mistakes that must avoid:

Linking issues
Keyword issues
Ignoring the Title Tag
Not using textual content
Using Flash technology and graphics incorrectly
Using unethical SEO techniques
Expecting immediate results
Avoiding SEO Maintenance

All the points are important to be consider while optimizing a site.

H1 to H3, Meta tags, Robots.txt files are also important in On Page Optimization of a site.

You will take a big risk if you submit your website to multiple search engines. It is a risk because it will make your site look like spam. If your site looks like spam it will not be ranked in search engine results. Thus, the opportunity to optimise your site will be missed.

Yes,i completely agree with you.nice points and thanks for sharing.