I think the easiest answer is as follows:
1) Platform - Chossing the wrong platform can hinder your SEO for years, make sure it has SEO friendly URL's, nice coding, unique titles, H1 etc.
2) Market & Keywords - Don't try and get to big for your boots, carry out good keyword research and go for the long tail phrases initially before you can focus on the bigger fish.
3) Number of keywords, try and keep each page optimised for 3 keywords or less avoid hundreds.
5) Avoid repatative text, write symantically use the "~" in google to find related words.
6) Make sure you don't block the spiders (Robots.txt) file.
7) Iframes - unless you have a valid reason then don't use them
8) Flash - don't make your whole site in flash, regardless of what addons they have to put text in noscript tags etc.
10) Keep it white hat, no SPAM, no hidden text.
I'll keep my answer to 10 points but the list is pritty much endless,