I've been using this program for years and recommend it to all my friends.  I'd say I couldn't be happier (which is mostly true), but there is one thing I've always wanted to see in this program:

Sliders.

Many times, I work with dozens of variables at a time, and it would be nice to have a slider interface to adjust each variable without opening the custom variables dialog.  Something similar can be achieved with the "Calculate" > "animate" option, but with a single variable.

I thoroughly appreciate the developer(s?) making such a robust and intuitive program free-to-use.  Thank you for all your hard work!