Great documentation, standard features, speed, good support, continuous updates, great reviews - are the main things to check through when choosing a WordPress theme.
This article also talks about a good tool to check your theme according to WP coding stadards - such as WP Test.
Design is often used as the main criteria for judging the quality of a theme. Of course this is important, but as the old saying “don’t judge a book by its cover” goes, this also applies to themes. Do your homework and dig deeper and make your own decision based on more than just how it looks on the demo site.
