Hey Ian,
Here's a couple of ideas I had. But first I wanted to tell you I really like the help section. Definitely makes a big difference.
1) When doing conversions the word "in" should highlight and maybe even caps. I don't feel it gets enough attention even though its the most operator in those calculations. Also when you're typing any kind of value that can be converted maybe a convert button appears on the far right of the line that the user can click and it brings up all the different values you can convert to.
2) Have the variables show up on the ride side of the screen so I can keep track of em and change them if I want. It would be cool if i could drag them onto the screen as well.
3) If I hover over a line that's been calculated a button should appear on the far right that is "convert to function". It then allows me to turn that line into a function with some clever UI. (It lets me name it and name all the parameters by replacing the numbers with parameters). EX: 5 * 10 = 50. If I click the convert button it would allow me to name it Area and change the 5 and 10 to side1 and side2 with the mouse. When I type Area it should suggest to the right the parameters. EX: Area (side1, side2) and as I enter each parameter it should bold the parameter I'm entering like a normal programming language. I think the key is to make it fun to build and use functions. If you allow people to build them through mouse clicks off of things they've already written you may get more people "converting to function" than would build it normally.
Later
-Joshua