I’ve used to write useless technical posts in the last 7 years. So, it inspired me to write about some of the biggest mistakes I did with my technical websites:
1. Copied from Stack Overflow
You spend hours trying to fix some bug and you finally find an awesome solution in StackOverflow. You are so inspired from the implementation that you decide to re-post the solution form StackOverflow in your own websites.
It’s useless, because your page will NEVER outrank the StackOverflow ‘s page. So, people will find the StackOverflow’s when they search for a solution using google. So, you’ll have an published post online no one finds and actually cares about it.
2. Just Repeat Technical News from Other Websites
There’s a new version of Visual Studio, and you just repeat the news from some of the leading technical websites. Just ask yourself, why a user would visit your website to learn the news instead of visiting TechCrunch for example!?
3. Try to Promote Your Own Product or Try to Promote Yourself
If the target of your post is to promote your product or just to promote how great are you, it’s a bad idea, because people hate advertising. You hate it too. Would you read advertising post?
For example, I released a new WordPress theme. If I write a post how great my theme is, would you gave a sh*t to read such post? That’s why I don’t write such post to waste both yours and mine time.
4. Write About Something You Have Junior Level Experience
For example, you’re a senior asp.net developer, but you don’t have any real experience/knowledge in php. Then you write a post like ASP.NET vs PHP. Your post will be incomplete; because you will miss some important information about PHP and php developers who find your post will hate you 
5. Write Too Much About How You Feel Using Some Technology
If you feel really inspired using some technology and you write about your feelings in your technical post that will be really a boring post.
The reason is that actually no one gives a sh*t about how you feel, if you are a geek/or not, etc. People will visit/read your website if they can find something they can learn/use in their own projects.