I'll give you my perspective from someone who buys themes from Themeforest on a semi-regular basis. I always pay for them even though I'm sure I could get some for free as I appreciate the time and effort into making them, plus the support if necessary.
A purchaser may only look at your theme for a minute or two, a bit like looking at books in bookshops.
1) Document the theme on screen very well. Far too many people use Lorem Ipsum far too much. You may know your theme inside out, a purchaser doesn't. Anywhere you can add text explain something, eg. "this is a blah, blah box - with this you can do X Y Z." "this is version 2 of the blah, blah box - it does A B C"
2) Have decent shortcodes. I like life to be simple and want a good shortcode generator as part of the theme. Far too many have the bare minimum.
3) Have lots of options. For example many themes have one style of blog - give 2 or 3 options out of the box, eg. large image, medium image etc.
4) Tag it well. I've recently been looking for a 'flat' theme, and before that a 'retro' theme. If it isn't tagged properly I won't be seeing it. Look to your competitors tags and copy them.
5) Document it well. Yeah, it's boring but I've just bought a theme where it seems like the author has given up halfway through. It has shortcodes I've never seen before and have no idea what the hell they do.
6) Provide great support. uDesign isn't the best theme on Themeforest but it probably has the best support of any theme. In contrast the theme I bought above the author isn't replying to me, or other people (for over a week).
7) Very few themes have gallery plugins or support of any significance. If you did you may set yourself apart. They all have portfolio options, but not galleries. Try and incorporate Justified Image Grid - IMO the best plugin around for images (no idea if he licences it).
8) As mentioned by another poster get into the blogs for example "Best Flat Themes" - there are typically one or two free and the rest are from TF. Write it yourself (with you as number 1) and offer it to a blogger.
IMO people WILL support good themes, it's the also rans and the poor themes that will struggle.
Just my 2p worth. Happy to review your listing if you send the details.