Windows Vista (and XP) have a fixed-size box for editing environment variables. It looks like this:
[Sidebar: I took that screen shot with the Vista "Snipping Tool". You draw a red box to define the capture area, but the inclusion of the red box in the screenshot was unexpected!]
So, when you want to enter something like a path (mine is 434 characters), you have to scroll this tiny box. And, yes, you have to scroll it by positioning the cursor in the box and holding down left or right (or using a keyboard shortcut like home/end). Once you set it, you have the same problem if you want to read it -- the form for viewing environment variables is fixed at a small width that appears to be designed for 640x480 monitors.
Some of the unpolished aspects like this, the buggy UAC system and slow printing, along with the the persistent awfulness of the calculator and notepad applications make me wonder why Microsoft didn't finish Vista before publishing it.