Some really minor tweaks to the function editor interface would make it much more functional (pun very much intended).
It's very difficult to see what you're doing in the function editor, and this makes it hard to spot a mistake. Currently, the function editor has a single-line text box, and if you're function is longer than the box (which isn't very large), the beginning isn't visible. To go back and proof-read your function, you have to use the arrow key on your keyboard. Even though that works, you can still only see a really small bit of formula at a time. I've sometimes tried to work around this by selecting my function and pasting it into notepad. But while you can write your function in Notepad (without the benefit of any of the functions editor's features to assist you) and then paste it into the function editor, you can't select and copy formulas you're composing in the function editor and paste them into notepad for proof-reading. The easiest way to proof-read is to create the function, then mouse-over it to view the formula. However, it doesn't stay up for very long and you have to move your mouse away and then back over the function and quickly find where you left off.
It's just not a good experience for a user.
Why not simply make the function editor a multi-line textbox with a scrollbar, and the ability to highlight, copy, and paste from/into it? Just this slight adjustment would make a huge improvement to the user experience!