I definitely agree with #1: targeting the wrong keywords. There's nothing worse than spending months targeting a keyword that has no real search volume to make it even worth it -- or that is so competitive that it would take you years to even have a chance to rank for.
Also, not doing proper onpage SEO is a big mistake I see a lot too. Proper site structure, titles, subtitles, metatags, tags, alt tags, administrative pages i.e. contact, terms, etc., anchor text, and so on are just a short list of things that you can do to make your site more SEO friendly.
And lastly, one of the biggest SEO mistakes I see are people relying too heavily on SEO. Remember: SEO is a metric that we never can fully control as it relies on too many variables that Google (an other search engines) implements and change constantly. As as we all know (or should know) is that what worked in the past may not work in the future. SEO is a ever-evolving game that you must be constantly willing to play to remain on top.