Personas
I’ve never been a big believer in personas. They’re artificial, abstract, and fictitious. I don’t think you can build a great product for a person that doesn’t exist. And I definitely don’t think you can build a great product based on a composite sketch of 10 different people all rolled into one (or two or three).
On countless projects over the years, agencies (especially) have supplied me with personas. To be honest, I've completely never understood the use of them.
Like most things, we'll make our beliefs fit into whatever contexts suits us. So if we want to build feature X, we'll use personas to motivate the decision. "Clearly, this is a feature that George would love!".
Whenever people ask me for directions on how to become a better user experience designer, I tell them to think of things that annoy them and try fixing them. It can be a really small feature of an otherwise great product - or it can be a product that doesn't yet exist. As long as it solves a problem that you're experiencing, chances are someone else is going to feel annoyed by the same problem.
So if you can’t design something for yourself, design something for someone you know. Get that person or people involved in your project early on. Basing your decisions on a matrix of personality traits isn’t what I’d recommend if you really want to build a great product.