I think this is quite an important question and the answer is always subjective. Though I think most people here would prefer Premium to be defined as 'kitted out' or just 'feature-packed' theme.
This comes down to your business model or business idea. Which is ... do you want to make a living from WordPress.
- would you prefer a free theme that has 1000's of installs
- would you prefer to sell a premium theme in very very (some times none) reduced quantities via a 3rd party theme site
- would you prefer to use this as experience for your CV ie getting a new job as a web-developer / web-designer
- are you paying to use someone else's framework for a theme? (cause this might make your theme's features a lot like others)
Some theme developers will focus on making free themes that are successful and they will give away all the features. Because free business is good business.
I don't think you should aim to just sell a premium theme on themeforest - that's not a viable option if you're just getting started in making a theme.