IDLE should be accessible and easy to use for beginners who have never used a command line. Therefore I don't think up/down should scroll through the history by default. Since IDLE looks like a text editor (even the shell window) it is more intuitive that the arrows should move the cursor around the window.

Note that a user can configure up/down to scroll through the history quite easily using the configuration dialog.

I agree that as it stands it is hard to discover how to scroll through the history, and that Return can be used to bring previous code into the current command line. I think we can find a better way to make these features known to new users, without making the interface less intuitive.

For example, we could have a "Welcome to IDLE!" screen that appears the first time IDLE is run by a user, which outlines a few basic features such as these. This should of course be complemented by better documentation, including a section on various commands (a separate issue).