These are the 15 Most Common WordPress Mistakes.
Now as you read them, some might be obvious if you are an experienced WordPress user, but this post is designed for those who are just starting out
Installing too many plugins.
Just install the Ones you need and that fit the CRUDS citeria. CRUDS stands for Compatibility, Relevant, Updated, Downloads & Support
Not optimising your blog for SEO.
This can be easily fixed by installing an SEO plugin, such as YOAST
Not backing up your site
Now this is easily fixed, if you are with a hosting provider such as my #1 recommendation
Not changing the default permalink structure