Working with many clients over the years, I've come to see a pattern in SEO mistakes; the biggest are:

Not putting your product and services front and center.

What does Apple do? They sell computers. When you go to Ford's website, you know they build cars. Their websites reflect this by being product-centric. Everything else on the website is a supporting factor.

Take a quick look at your website's navigation. Does it have "home," "about," first? Does your website easily explain what you do? What and where is your elevator pitch? If you didn't speak english, would you still understand your website?

Being everything to everyone

Don't be a jack of all trades; search engines and users need to understand what you do. Keep things clear and concise. If you have different products and services, clearly separate them, or create individual websites.

Honda Manufacturing does this perfectly, you know they produce cars, motorcycles, ATVs, airplanes..., and they have them clearly silo'd on the website.

Mismatched Search Intent + Keyword Stuffing

Name your website accordingly; if you are known as "Isaac Consulting," don't buy the domain name "BestSEOconsultantOttawa.com," it's misleading and unbranded.

If your webpage is discussing a topic, don't throw in a bunch of related words hoping it will help you rank, Google is smarter than that.

In real world terms, it's no different than trying to get my friends to call me "The Napster," or adding "Ninja, Guru, Synergy" to everyday SEO talk. You don't sound any smarter, you just sound like a douche. My name is Isaac, and I do SEO; people will respect that more. Same applies to websites.

Not getting to the point.

Don't be like a food blogger who insists on telling you their life story before getting to the recipe. People visit your website for answers, solutions, or information.

When you are writing, remember two of the most important qualities: WIIFM (What's in it for me) and WIIFT (What's in it for them)!

Stay tuned as I expect to add and grow this list.