I'm not sure if the auto-compete API stuff would prevent any of the following, but after using the auto complete (via CTRL+[space]) for a while, I have the following wishes/ideas:
1) Any chance the [tab] key could work like the [return] key and accept the currently selected item in the completion list.
2) I think it'd be very nice if you could continue to type while viewing the auto complete list and have it do one of the following:
2a) Select the item in the list. So if the list shows "red," "green," and "blue" then hitting 'b' selects blue in the list while leaving all other options shown and not writing anything to the main edit window.
2b) Narrow the list down while adding to main edit window. So if the list shows "red," "green," "blue" and "blunt" then hitting 'b' adds the character 'b' to the edit window and also narrows down the list to show "blue" and "blunt".
Personally I'd prefer 2b but have a suspicion that it might not be so easy if other stuff can hook into the API and you're delaying with snippet selection etc. I consider either nice and would prefer them to the current method, though.
Lastly I want to say how great it is that jps responds so quickly to people's requests and gets out so many betas. Know that I love the editor even if you never coded another line, Jon!