What makes a good WordPress theme?
The most important factor of any WordPress theme is the level of customization. Many themes look awesome right out of the box. You install it, activate it, and suddenly you have a professional-looking site. However, it's YOUR site and you're likely going to want to make many changes over the years that you spend with it. For this reason, it's extremely important that you pick a theme that can grow with you. The more customization is the better. When looking for a theme, because they're often so customizable, you shouldn't just take them at face value. In other words, one screenshot or even one demo site probably doesn't do the theme justice.
What you should be paying attention to is the general structure and look, and the customization options that you have at your disposal. Many times, I have seen a theme made for a service provider with an image slider homepage design, but then that same site can easily be transformed into a blog layout with posts dominating the front page. This is why you should concentrate more on what the theme can do then what it currently is in any demo.
Do I have to Pay?
A very common dilemma for new webmasters is whether or not they should pay. If you are strapped for cash, you can still get a really nice looking site with plenty of customization options absolutely free. You should have no problems getting started without cost. That being said, the paid themes you can find online from sellers, like Elegant Themes and Woo Themes, for example, look better and have much more expansive customization options.
These premium WordPress divi themes often have an extraordinary amount of ability to transform and change to meet your needs. I wouldn't recommend installing a theme designed for an eCommerce site and then trying to use it for a blog, but at the same time, you can do almost anything with most paid themes. If you're new and don't want to spend any money, go with a free theme. If you're ready to drop the cash, which you probably should eventually, keep these principals in mind and get the most bang from your buck when looking for the best WordPress theme.