I'm gonna be a bit harsh, but I hope you take this message to heart to improve yourselves in the future (if you ever read this, WoC).

I personally stopped at "the biggest issue was that we were missing some data validation in our files. Make sure you use the esc_ and sanitize_ functions provided by WordPress!” which was a direct no-no to me.
Basically, if not thanks to the professionalism and thorough review of Envato (I didn't think they'd go this far in their review tbh), you would have given your users a theme with security holes in it (that doesn't mean that there aren't any left either).

We're in 2015 folks, the era of cyber-criminality. When you make web-based stuff, you CAN'T even for a single second leave out the security aspect to focus only on design and functionality. Your theme is a tool that interacts with databases, potentially storing information that could ruin people's lives if leaked in the wild. You need to understand that you have an very important role in the privacy chain as a theme seller. And no, what I'm saying is unfortunately not paranoid, it's the harsh reality of the Internet nowadays.

Now, some people are willing to forgive, some are not. I personally cannot trust a person that delivers their first product with a critical flaw in it (and security is a critical issue), so I probably wouldn't consider your products in the future. Again, that is just my two cents so obviously don't take that for a generality. Fact is, their are many developers in the same boat as you on Themeforest (and it's a shame :( ).

Now, it would be rude of me to just rant without giving you any pointers on how you can improve the security aspect of your products, which would not only please your customers but give you peace of mind as well (which is just as important and gratifying!).

- Have a thorough read and understanding of the Top 10 security vulnerabilities you have to look out for :
https://www.owasp.org/index.php/Top_10_2013-Top_10
- Perform systematic "penetration testing" on your products (you can look it up on the web) for both the front AND the back ends. You will eliminate the majority of problems there.
- Audit your theme (the front end) with tools like Sucuri and the like.

These tasks are time consuming and require a lot of patience. However, by doing this you will greatly increase the trust of customers in your products. And nowadays, trust takes a huge part in the "successful business" pie.

Best of luck to you guys! As far as UI and UX goes, that's great stuff.