Seriously, what makes a theme great? Good design? Yes, but I've seen many WordPress themes that almost made me cry because of their beauty. But when I bought and downloaded it, I saw that it lacked even the basic coding standards and it immediately broke my WordPress installation.

So, is code structure important? Sure, but I've stumbled upon very sophisticated code in WordPress themes - the beauty of object-oriented PHP, readable CSS, brilliantly documented code blocks, and so on. But it broke the minute I installed it on WordPress 3.6 - it seemed that it only worked up to version 3.2.

What about maintenance? Of course, but this one time, I had to buy a theme for my client that utilized the latest WordPress features for that version. It was a good theme, but I just couldn't move a block to another place - it didn't even let me avoid using it! The website had to be the same as the demo website or it would fall apart. So, flexibility? Yeah, but...

You get the idea - there's not just one thing that makes a theme great.

Customer care, valid code, secure code, compatibility with major plugins, and it has to be distributed properly with respect to the licensed products it includes.