It's like coding in an adversarial text window -- the font isn't monospace, there's no syntax highlighting, no auto-indent based on parenthesis, and when you accidentally hit enter (instead of ctrl+enter) you leave the text editor (and then it'll pop up with complaints about your function not being parseable). Also, dragging the size of the edit box bigger means you won't get much of a view of your spreadsheet -- it would probably be nice to attach the edit window on the right (if it was turned into a proper formula editor).