Well, as the title says, what makes a wordpress theme "premium"? I see a lot of threads on here for premium themes, but a lot of them are at the same quality as those which aren't advertised as premium, so, where's the difference?
Premium themes generally means themes that cost money or require license to legally use it on your site.
There are some wordpress themed that have been marked as "premium" but they aren't really premium (that's why they look like any other quality theme). If you are looking for true premium wordpress themes, you can automatically see that the quality is much higher than the average.
Yeah a lot of them are mislabeled but generally speaking anything that stands out tremendously from the default theme or accommodates a feature that is needed but not featured in many themes could be considered premium.
Well, everything the others covered above there. Costing money, quality, uniqueness from any other themes out there, features, being able to have customizations done to the theme at buyer request, and whatever else you could think of, that makes the theme(s) / designs different from most all others out there. Mainly, uniqueness and that it generally cost money, either by a straight sale / auction or licensing fees.
You can't just go by the look of the design. If it is worth paying for then the template creator has taken time to code it correctly so that there are not many issues with different web browsers or bugs using it within wordpress and any other plugins